Edit: @HollyswordFor example, a Razorback is transporting a Tactical Squad of five models and one Space Marine Captain when it is destroyed. The Razorback rolls a 6 for its Explodes ability, inflicting D3 mortal wounds on each unit within 6". The six models inside now disembark, and six D6 are rolled. Two of the results are a 1, so two models are slain – the Space Marine player chooses two of the Space Marines from the Tactical Squad. The Razorback model is now removed from the battlefield.
Perfectly in keeping with lore. Chapters go renegade for all sorts of reasons, and as for the mutations, check out the “Cursed” 21st founding, e.g. with the Black Dragons chapter, who have large bone growth mutations.