If Aethelred had given Aethelflaed a chance to be a partner and friend, he would have achieved his ambition of becoming a true King of Mercia. However, he died without achieving anything significant despite having lots of opportunities to do it. He went down as the worst Saxon leader in The Last Kingdom. Especially when he died, leaving Mercia in crisis.
Before his death, Aethelred apologized to Aethelflaed for mistreating her, claiming that he knew how it felt to grow up without love. The show never dived into how he was brought up, but it must have been hectic if his behavior in the show is anything to go by. He was an ambitious leader with the wrong motive, always seeking to become greater by exterminating those around him.
Raping Aethelflaed
When Aethelflaed saw the man she was matched with, she was relieved because it wasn't an old man that would probably make her marriage unhappy but she was wrong. Having grown up training to be a queen and a partner for her husband, the least Aethelflaed expected from Aethelred was just a little respect.
Instead, on the very night of their wedding, Aethelred turned into every woman's nightmare raping her yet she couldn't leave him because she loved Wessex and Mercia more than her own happiness. It was the worst marriage in The Last Kingdom and the reason why her marriage and Aethelred's reign was ruined.
Making An Enemy Of Uhtred
Uhtred is the bridge between Saxons and Danes with friends on both sides of the divide. Aethelred being the leader of the kingdom that shared a border with Danes, needed Uhtred more than Alfred and Wessex did. He instead hated Uhtred from the first time he met him.
He started the enmity by insulting Gisela without any provocation. His anger and envy of Uhtred increased when he realized that everyone was praising Uhtred and that Alfred wouldn't allow anyone to harm him. If he looked at Uhtred as a potential ally rather than just another favored Dane, he would have had a great ally to fight for Mercia as Aethelflaed has.
Bringing Aethelflaed To London
Aethelred turned into an animal against Aethelflaed on their wedding day, which was strange because Aelswith and Alfred had accepted him as a perfect gentleman. He never took the time to understand her and take advantage of her leadership skills learned at Alfred's side. He tormented her with the threat of breaking the peace with Wessex if she didn't submit to him.
He forced her to come to London in a battle against an enemy he didn't understand. His chauvinism led him straight into Sigfrid and Erik's trap leading to Aethelflaed's abduction. Even after Uhtred rescued Aethelflaed from Erik, Aethelred never trusted her again, so it effectively ended their short-lived marriage.
Stabbing Aldhelm
Aethelred proved himself as an unfit leader when he ordered Aldhelm to seduce Aethelflaed so he could divorce her for infidelity. Aldhelm's loyalty was always with Mercia and not Aethelred, which is why he found it safer to defend Aethelflaed rather than help Aethelred in his mess. Aethelred never realized just how lucky he was to have him at his side, though.
When he refused to send for men to help undermine Edward's crowning as king, he understood that Mercia was only strong if it allied with a smart leader in Wessex, and Aethelwold wasn't that person. Aethelred chose instead to stab him, forcing Aldhelm to switch loyalties.
Plotting To Kill Aethelflaed
Unity of Mercia and Wessex is the key to achieving a united England out of the four Saxon kingdoms, but Aethelred wasn't interested in England, just his personal gain. When his marriage to Aethelflaed failed, he didn't understand the need to stay close to her.
Plotting to kill his own wife after he himself pushed her away was just too evil of him. If Aethelflaed had disclosed the plot to Alfred, the peace between Mercia and Wessex would have ended, which would have meant the end of his rule in Mercia as well. The attempt also served to push Aethelflaed closer to Uhtred, which meant Aethelred had lost her forever.
Trusting Eardwulf
To Aethelred, Eardwulf was Aldhelm's replacement, but eventually, his doom. After pushing Aldhelm and Aethelflaed away, his circle of trust was finished leaving him vulnerable to liars. Eardwulf was only interested in the glory of his disgraced family; he didn't care about Aethelred and Mercia.
He made the mistake of trusting Haesten, a liar everyone knew. Aethelred didn't even care to crosscheck the source of information before invading Daneland. Eardwulf also preferred to hide his mistakes from Aethelred rather than save Mercia. He was the worst Mercian in The Last Kingdom, after Aethelred himself, of course.
Abandoning Mercia
Mercia shared a border with Danes, which is why Coewulf sought an alliance with Wessex in the first place. He knew that Mercia couldn't survive if a strong army didn't defend it. After Alfred helped him secure the border for all those years, Aethelred was stupid enough to leave it unguarded at the word of Haesten.
His ambition and hatred of Edward made him stupid as he sought to prove himself as the deserving king of the united England. The saddest part was that he left with all the soldiers leaving the whole kingdom at Cnut's mercy, making him the worst leader in Mercian history.
Rejecting Aelfwynn
Aethelred was a womanizer, but Aethelflaed was his rightful wife meaning, any heir to Mercia would have to come by her. Aelfwynn's paternity is still in question, and Erik the Dane may well be her father, but Aethelred doesn't know that. Even with the paternity question, Aethelred should have recognized that Aelfwynn was an innocent child and the only legitimate child he had.
Instead, he chose to dismiss the little girl, calling her a Half-Dane as an insult to Aethelflaed. It only made him as bad a father as he was a king.
Failing To Plan For Succession
Being a king comes with a responsibility to the throne that Aethelred never seemed to understand. When Coewulf named him as the match for Aethelflaed, he was entrusting him with Mercia's future. Aethelred, on the other hand, never grew out of his teenage stupidity as he continued to sleep around with women and failed to plan for the future of the throne.
He had distanced himself from his family and didn't even have an idea of who could succeed him or be matched to Aelfwynn when he died. His failure to plan for the succession caused the crisis that nearly got Aelfwynn killed as Edward and Aethelflaed struggled over who would rule Mercia.
Mistreating Eadith
Eadith was Eardwulf's sister, whom he used to manipulate Aethelred, but Mercia's confused king never noticed it. While she was wrong to agree to her brother's plan, she still cared enough about Aethelred not to plan his death. Aethelred, on the other hand, treated her as nothing more than another mistress, hurting her feelings without noticing.
When things got bad, and Aethelred was on his deathbed, he had no more friends; Not even someone that could listen to his last message. While Eadith was there, she hated him even on his deathbed and only watched as her brother killed him. In her opinion, after mistreating her, Aethelred was no better than her brother, which is why she didn't even mourn his death.