There are two issues; the assault/battery, and the injury.
The assault/batterywas wrong, regardless. The penalty for that wrong should, in part, be based upon the injury caused.
If the injury was self-inflicted, then that fact would affect the degree of the penalty, but would not negate the fact that the assault/batterytook place, and should still be punished.
Now, my question to you: since there is no evidence that the old man wanted to risk cracking his own head open, is it still your opinion that the officers that pushed him did no wrong?
And if that's not your opinion, what is?