So what happens if he calls the police? They take 10-15 minutes to get there, the guy is long gone, likely will be arrested at some later date robbing another household.
Let's compare people here. You have the student, who has no previous criminal record, is working to contribute to society, and hears a strange noise in his garage. Probably not wanting to be paranoid and call the cops over possibly nothing, he arms himself (in a slightly ridiculous and awesome manner) and goes to check it out. Sees a guy, says something, gets attacked. He doesn't know if the guy is armed or not. There could be a concealed gun or easily a knife. Student freaks out and kills the guy.
This criminal has been in and out of jail so many times, and this is what, a week after he was out for his last offence. Clearly there is no hope for rehabilitation for this guy, and he'll just commit another crime if he wasn't killed. You want to make your life one of a felon? You live (or die) with the consequences.