He is the person (whom someone killed). — Someone killed HIM.

He is the person (who was killed). — HE is the person who was killed.

The aid came from Mr. Jarvis, who ironically was the father of Aurthur Jarvis, whom Absalom had killed.

Absalom killed… he did the action..he can be referred to as WHO. But Arthur Jarvis did NO action, he received the action (a knife in the back), therefore he is WHOM was killed.

Who killed Jarvis? Absalom.

Whom did Absalom kill? Jarvis.

“Who” can also be neither a subject or object, just a question word. Do you know who Absalom killed?

However, that sentence has some structural issues. You may wish to break it up into two sentences, becuase it kind of has a run-on feel.

Ex.

Stacy, [who/whom] likes pineapples, is hungry. — Ask, “Who likes pineapples?” Answer: She likes pineapples. The right choice would be who in this case.

This is how I remember it — if the pronoun has a consonant at the end (him/her), it’s whom, which also has a consonant at the end.

Ex.

She gave it to [whoever/whomever] wanted it. (She gave it to [who/whom]? She gave it to HIM. Answer = whomever)

If the pronoun ends with a vowel (he/she), it’s who, which also ends with a vowel.

Ex.

Jimmy, the boy [who/whom] lives across the street, is 10.

(Who lives across the street? HE lives across the street. Answer = who)

So in your case, ask, “Absalom had who killed?” HIM. Therefore, it’d be “Arthur Javis, whom Absalom had killed.”

(You need that comma there, by the way. Unless the fragment is necessary for pinpointing which person you’re referring to, it’s considered an interjection and needs to be offset.

Ex.

“The boy who has red hair is cute.” — “who has red hair” is necessary to distinguish him from the other boys who have different colored hair.

vs.

“John, who has red hair, is cute.” — “who has red hair” in this case is just adding detail to someone who is already pinpointed.)