Brutality on that level and a We Have Reserves attitude from even the front-rank troops is very intimidating. These things are bigger and tougher than any human, and if something so much stronger (or looks like it is, at least) than a human can't stand up to what the enemy is dishing out, then the soldiers may start to wonder what chance they have against the invaders, especially the inexperienced Rookies.
While it has been removed, the Will penalty for Mind Controlled aliens dying might make sense for the nastier ones.
