If the guy was on the ground, surrounded and being kicked, he very well could be killed.
For you to 'step in', to attempt to discourage them from further injury to the guy is justified.
'step in' to what degree is what we are concerned with though.
'Step in' could mean:
A: A verbal confrontation. "Hey, stop that! I've called the cops!"
B: A verbal & physical confrontation. "Hey stop that or your going to have fight me now"
C: physical confrontation. -Push person 1 into 2, break #3's knee, and then worry about #1&2.
however A B or C work out, the conflict could raise itself to deadly force based on the actions of the assaulting individuals.
I.E. You say "Hey stop that!" The assaulting kids then turn and begin to walk towards you in an aggresive manner. You say "Don't take another step, I am armed"
If they continue, you draw.
If they continue, and close within distance to disarm you, you shoot to kill.
DISCLAIMER: this is all just my opinion.
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FS