how to get a specific ending? *spoiler*

[Avalon4]

I got a different ending on that path -- If you destroy the three chimera which defend SAM, he will try to run away. If he leaves sensor range you get a somewhat peaceful ending (but which ends with the common scene of transports being wiped out by a symbiont army)

Likely you just have to destroy the Manticore for that ending... I suggest coming from above or below SAM, behind his guards, if you can, otherwise they end up right between you and him -- then fly Alice in with 4 Supernovas (with Deadly Missile I believe it is well over 75,000 damage per full volley). Before the bombs hit though, use Depolarizer on SAM.

An interesting variant of this is, using MIRV-2 Reapers instead of Supernovas. These missiles do equal damage (if the Reaper only hits one target) but since you can carry 3 of them per launcher you can put out more damage, more quickly