While Tim Drake has often been held up as a potential successor to Batman, the current Robin proved he's not suited for the role by actively plotting the murder of a villain. Some day Bruce Wayne will have to retire from the mantle, and when that day comes he will need a successor. Fans have debated constantly about who it could be — Dick Grayson, Damian Wayne, Terry McGinnis, Cassandra Cain — but the one thing fans haven't debated is who it shouldn't be, and Tim Drake belongs at the top of that list.
Batman has had a lot of would-be successors over time, from Dick Grayson to Damian Wayne, even Tim has worn the Batsuit in the main universe during the "Battle for the Cowl" storyline. But overall the general successor is either assumed to be Dick Grayson or Terry McGinnis, who acts as Batman in the Batman Beyond timeline of comics. In the 666 future timelines, Damian Wayne also takes on the role of Batman. Even Jean-Paul Valley has acted as Batman for a time, though he ended up being a fairly dark one and Cassandra Cain has made it clear that she wants to become Batman.
Readers are shown exactly why Tim shouldn't be Batman in the storyline "A Lonely Place of Living," by James Tynion IV and Eddy Barrows. Taking place in Detective Comics #963-968, this storyline shows readers a Tim Drake from a dark future where Batman is killed by Batwoman and Tim takes up the mantle, taking over the city of Gotham and keeping it under his iron rule. One of the things Tim does to make sure he contains control of the city is using the gun Joe Chill used to kill Bruce's parents, to kill criminals. This new version of Tim Drake takes the name of Savior. While the history of Tim as Savior is undeniably interesting, what's more interesting is the picture it paints for Tim if he took over as Batman. The idea of a future Tim becoming a murderous monster isn't necessarily a bad reflection on Tim's character. After all, there have been plenty of versions of Batman that ended up committing murder. The bigger concern is when Tim begins exhibiting murderous intentions in the current timeline, which he has done.
Tim Drake Once Tried To Murder Captain Boomerang
Tim fought for the chance to be Robin, doing absolutely everything he could to prove he was fitted for the role, and he does believe in Batman's rule of not taking a life. But in the story "What Goes Around..." from Red Robin #26 by Fabian Nicieza and Marcus To, Tim Drake manipulates a situation where Captain Boomerang will get himself killed. A failed heist here, a bad move there, and eventually he will be killed by another criminal. Tim justifies this by saying that if Captain Boomerang just doesn't pick the bad option, he won't die, so it's not really murder. But when his plan starts to come to fruition, Tim pulls the failsafe, because in his own words, "I couldn't let someone else kill him and I couldn't let him kill himself. So that had left me with only one choice" In the end Tim decides not to go through with it, saving Captain Boomerang and sending him off to prison. But Tim still set this all up just to kill one man.
While Tim does back off from enacting his plan on Captain Boomerang, the fact he constructs the plan and puts it into motion at all doesn't speak well for the darkness inside of him. When this event is combined with Robin's future as Savior, it becomes clear that Tim Drake can never be Batman.