Well, if you play on hard mode as-it-is-ATM, it'll be one of the baddest skills they could use... you know, HP isn't really the problem, the problem is that you get killed by about two hits from each enemy... and guess what two slimes do rather than a single one? Yeah, kill you twice as fast :p (you better find another hard-betatester for the Part I XD ).
For the kill-when-HP-are-0-thing... if you have the slimes split everytime you attack them with a blade or something (so if it wouldn't be a skill they use, but more likely a effect that'd occur if they split too much), it'd be more logical if they'd just kill themselves rather than 'not getting split' when they're at 1 HP... if they do this intentionally, though, it'd be a bug ^_^