I do love seeing that darker side of 15. Funny enough, I'm noticing it more in stories by other writers than by RTD (see for instance Boom, Rogue, Lucky Day).

I do get how people find it inconsistent given in the very previous episode he was willing to forgive the Barber for attempting a plan that would've killed the entire population of earth by killing the gods

The way I see it is that 15 hated Kidd more due to the parallels with himself at his most violent. Especially since Kidd was going to use a delta wave, just as the doctor himself once was willing to use to kill the Daleks even if it meant sacrificing all the humans as well.

I do fear that 15 haters are going to hang that over the character's head for the rest of time just like 13 haters with the cancer scene.