How would I write "They killed them" ?
HoH. There is some ambiguity in the null prefix (the absence of a prefix). You could add pronouns to make it more clear: chaH HoH chaH. It doesn't clear it up a lot, but it is no less clear than "They kill them."
Did he miss a lesson?
We don't currently teach the word for owl (lIr) in this course, but I've only seen Duo alone, so it would just be HoH (or HoH lIr).