Use your mouse/ fingers/ other pointer device to drag the bad knights away from the small monster until it will be able to get into their heads.
As they're slowly loosing their minds, they'll start to attack each other (turnig red), and if picked target is not going to be killed, they'll get completely mad and finally kill themselves (for bonus points and additional life!).
Liked the mind control idea. You should consider implementing it as special ability with a cool down and stuff rather than the knights just gradually loosing their minds over time. It would diversify the gameplay a bit and give the player more control over how they attack.
The idea is great :) Also the gfx is cute and not too bad! Only critic, it is kinda easy. Maybe it is getting much harder after some time, but the first five minutes were pretty easy.
click fest