Which sentence uses the word who or whom correctly?

1_Whom was the old man that the three rioters met?

2_The three rioters went in search of Death, who they kill.

3_Against whom did the two rioters plot?

4_The apothecary sold poison to the young rioter, whom used it to kill his friends.

I have NO IDEA.. I'm trying to help my younger sister.. but I'm not good with whom& who I usually use which one sounds right to me.. eh.