That's just part of the game. The bots are by far the worst offenders during FFA tournament battles. Nothing stopping you from returning the favor. 

It works both ways. Any time you see a low health mech and kill it you may have "stolen" a kill. It is frustrating but not particularly a problem as far as I know. 

Also FFA rewards you significantly for damage, so even if you get a few kills stolen you still get points for the damage you caused. 5v5 though (both regular and tournament) only count the number of kills and assists so there the stolen kills impact the score more.

I target low health over full health when convenient. 

Yeah, that’s always frustrating. If you did most of the damage, it definitely feels unfair when someone jumps in for the final shot, but unfortunately a lot of games only count the last hit for the kill. An assist or damage-based reward system would honestly make things feel much more balanced.