So as I see it we have the following choices and potential outcomes:
1) We engage and take a shot we are not confident in.
(a) We strike an innocent.
(b) We get lucky and strike only the bad guy
2) We do not engage.
(a) He stops shooting and walks away
(b) He continues to shoot people until he runs out of ammo or targets.
It seems to me that the high favorability does not coincide with the high probability. So it comes down to personal morality in the risk assessment.
If we engage there is no doubt as to our responsibility for outcomes 1) (a) and (b). Historically actively shooters have been known to surrender or to kill themselves when first confronted with effective resistance. So if we choose not to engage does that necessarily mean we are not responsible in any way for outcome 2) (b)?