There's no more getting around it: The Walking Dead has officially made Negan a Whisperer. So far, The Walking Dead season 10 has been focused on building up to the Whisperer War between the titular villains and the communities of Alexandria, Hilltop, and Oceanside. This included Alpha sending Dante to infiltrate Alexandria, which ultimately led to Siddiq's death and the death of numerous others at Beta's hands later on. But that was only the beginning of their plans.
Alpha and the Whisperers managed to secure a walker horde inside a cave, which is what the people of Alexandria and Hilltop feel the most threatened by. Given that those communities aren't able to withstand large scale attacks without the right number of people and the proper weaponry on hand, both of which they don't have, it makes sense that they've been trying to find Alpha's horde so they could destroy it. Unfortunately, they found it too late, and now a part of it has been unleashed.
In The Walking Dead season 10, episode 11, "Morning Star", Alpha sent a small portion of her horde to Hilltop to destroy it. But before this, she inducted Negan into the Whisperers after previously having sex with him. He even got his own Whisperer mask. This is all starkly different from the comics, in which Negan killed Alpha instead of joining her in her crusade. What's curious is that Negan took this as an opportunity to save the people of Hilltop by employing an old Savior strategy. And it worked, but not in the way that he hoped.
Negan suggested to Alpha that she get Hilltop's people to bend the knee and join her instead of wiping them out. It appears that he even suggested she block their exit out of Hilltop with broken down trees on the road - something he and the Saviors employed in the controversial Walking Dead season 6 finale that led to the Saviors taking over control of Rick Grimes' group - which Daryl confirms must have been Negan's plan when he said, "This is Negan." While it didn't have the effect he wanted, it still forced everyone to return to Hilltop.
Ever since Negan was let out from Alexandria's prison, he hasn't been as murderous as his former self. At one point, he killed the person that released him after that guy killed a mother and her son. He wanted Alpha to induct the people of Hilltop into the Whisperers, but since the Whisperers see themselves as being one with the walkers, destroying Hilltop and turning its inhabitants into zombies is Alpha following Negan's advice. Perhaps this will get him to change his mind and kill her when the opportunity presents itself. But for now, it looks like he's fully on Alpha's side.