WMG / Batman: Reptilian

This incarnation of Killer Croc is an Adaptational Jerkass with none of the sympathetic qualities of his mainstream counterpart.

Which explains why Batman seemed to take pleasure in his suffering and later on, show no empathy for Croc being locked up by the government to be experimented on. While we're not fully told or shown what Killer Croc's criminal history is like in this incarnation, one possible explanation could be that unlike his main DC counterpart, this Killer Croc never showed kindness or empathy for downtrodden people and Batman never got to see his gentler side, instead only seeing Croc commit atrocity after atrocity without remorse which eventually numbed Bruce. Considering how many other incarnations of Batman don't take too kindly to other people attempting to kill or torture his villains for the sake of science, this Killer Croc being a far worse person than his Pre-Crisis, Post-Crisis, New 52, and Rebirth counterparts is a legitimate possibility.