Hmm. After being put through all that, it would be weird NOT to lose your temper.
Literal
Hmm. This-much thing [obj-を] done-[passive-されて], snap-not direction [subj-が] strange.
ブチキレる 'to snap / lose one's temper violently' is an emphatic slang compound — ブチ (violent intensifier) + キレる 'to snap.' ~ないほうがおかしい 'it would be strange NOT to X' inverts the judgment — reframing anger as the expected, rational response. されて (passive of する) marks the speaker as a victim. うむ is a ponderous, masculine thinking-aloud interjection.