I've found this definition and a few others regarding murder, many of them being similiar: "The unlawful killing of one human by another, especially with premeditated malice."
The common words in all the definitions is "unlawful", "premeditated", and "malice". Ask him to elaborate on his thoughts, and if you cannot come to an acceptable answer, call him a moron and drop the class, seriously (well, not necessarily the moron part).
I once had a physics teacher who tried to use an elevator's motion in an example in class. He stated that when you go down in an elevator you will feel heavier. Conversely, when you go up in an elevator you will tend to feel lighter. The frightening part is that everyone in the class just wrote it down, which is where the term "sheeple" comes from. Anyway, 20 minutes later and with some frustration I had to teach the instructor about his lesson. I don't mean to sound arrogant about this, but if I have a teacher that doesn't know the basics, then he/she shouldn't be teaching. One other time I got up and walked right out in the middle of the lecture because I thought the teacher was full of... well, you know. But hey, I figure if I'm paying $800 dollars to take a class, I'm entitled to do that.
Hmm, didn't mean to hijack the thread, sorry about that. Anyway, I say stand up to you teacher and get him to explain himself. I always recommend being respectful even if you do feel frustrated though. Feel free to tell him that you feel his answer is unacceptable to you, unless he has proof or good logic and reasoning behind his statement.
Also, if you are the only one that thinks the way you do you aren't necessarily wrong, but keep an open mind because you probably are.