Even when there are more than two parts to a compound sentence, use a comma before each conjunction.
If you want something choppy/abrupt but with correct punctuation, you can write the example this way, without the commas and conjunctions: Aliens abduct a human. The human is a serial killer. He’s less than amused.
Aliens abduct a human, but the human is a serial killer, and he’s less than amused.
One thing we can tell from this snippet is that the abductee is highly intelligent, which probably means the aliens are in deep trouble… Wanna know why there’s a correlation between high intelligence and being a serial killer? It’s not, as some idjits may tell you, because highly intelligent people are especially evil and/or prone to violence… although constantly hearing other people say that high intelligence is evil may cause a few of us to snap. Um. Anyway… The correlation, such as it is, exists because would-be serial killers who aren’t highly intelligent get caught before they can kill enough people to be classified as serial killers. It’s that simple.