If the guy had had an explosive on his back or around his waist, kicking it would be suicide, and as PPs have explained, stamping on a hand clutching a knife that's still dripping blood from its last victim a few minutes ago is a good way to get slashed yourself. Even if he did have his hands out for stamping, which he was refusing to do.
I'm an absolute fucking wuss, I'd get pulped if I was ever in a fight, and even I can understand that when you're charged with risking your life to protect the public from an armed, maniac stabber who's refusing to comply with orders and might well have a bomb on him, and all you've got is a taser and truncheon, you're going to have to do stuff that the public doesn't often see (thankfully) and isn't going to enjoy witnessing. Doesn't matter. It was entirely necessary, it's the reality of subduing violent criminals, and thank God they did it. They could have been stabbed or blown up for all they knew, give them all medals the size of bin lids. Heroes.