Batman has always struggled with the line between justice and vengeance, but the finale of Titans sees him go more murderous than ever. Virtually every iteration of Batman has killed, even the versions that have a strict no killing rule have been responsible for the loss of life, either incidentally or by allowing someone to die instead of saving them. But the Titans finale reveals a Batman that sheds all theat nuance and turns into a straight up no-nonsense murdering machine.
While debates have always raged around different Batman stories as to whether or not Batman actually pulled the trigger, if an off-screen death counts since it can't be verified, or if Batman is guilty of manslaughter when his collateral damage results in loss of life, but with Titans' Batman throwing Joker off a building and slashing throats with his batarang, there's little room for discussion about exactly what his intent was.
Fortunately, this version of Batman is simply a dream of Dick Grayson, but it does contrast against other iterations, such as Batman in Batman v Superman: Dawn of Justice to show what would happen if Batman were actually a mass murderer. Over the course of the episode, there are 52 deaths Batman was deliberately responsible for, including some fairly major characters.
The Joker is Batman's first victim. At first, he threw him off a building and onto a car. When that didn't kill him, he broke into the hospital and stabbed him in the heart with a Batarang.
It's not clear exactly how Riddler died, but an arm covered in question mark tattoos can be seen under a tarp on a gurney in Arkham Asylum.
Also in Arkham Asylum, Two-Face's hand is seen holding his signature coin, all covered in blood.
The Ventriloquist's puppet, Scarface, can be seen on the floor of another Arkham cell, alongside presumably the Ventriloquist himself.
Captain Frank Finney
Frank doesn't seem to be based on any particular existing Gotham Police characters, but that doesn't mean his death doesn't count just as much. Frank is brutally killed by Batman during his assault on the Batcave.
In Dicks' dream, Starfire is an FBI agent and serves as a liaison for GCPD's attempt to take down Batman. She's gets to him after he's already taken down Frank and the SWAT team, and is about to incinerate him, but he blasts her with a cold gun. Freezing her solid.
Often depicted as Quincy Sharpe, but unnamed in Titans, the warden of Arkham is also murdered by Batman. The reasoning isn't explained, but Arkham isn't rehabilitating the psychopaths that get locked up, and they also presumably keep "escaping," so Batman holds the Warden responsible, intentional or no. Apparenly he does a number on him too. Starfire "his own family wouldn't recognize him."
Cop at The Hospital
When Batman's first attempt to kill Joker fails, he has to break into the hospital to finish the job. We see one police officer on the ground in the hallway, but there were likely more off screen.
Arkham - 8 Inmates
Some Arkham inmates are easily identifiable by some sort of trademarks, but there are numerous bodies and body bags littering the facility. A total of 6 body bags and two other dead bodies are seen as Dick investigages the crime scene.
Arkham - 15 Staff
Like the Warden, Batman also holds the Arkham staff liable. After all, Harley Quinn was once an Arkham Doctor. Batman surely sees them as major contributors to the failings of the asylum. There are at least 15 bodies seen under sheets, but there could be a few more off-screen.
22 GCPD SWAT Officers
They may have made a big mistake invading Batman's lair, but Batman was far from forgiving of Gotham's finest. Slashing and stabbing with a batarang, Batman brutally ends the lives of numerous SWAT officers. It's hard to count just how many go into the building, but there are 22 little lights on Dick Grayson's HUD that get snuffed out when Batman attacks.