That's the fallacy of trying to understand criminal acts. For the most part, if someone were as smart, logical, and thoughtful as you are when you imagine the best way to commit murder, the kind of person to actually try and commit the murder would not be as smart, logical, or thoughtful to have gotten into that situation in the first place.
There are exceptions, of course, but it's enough of a possibility that it's probably better to take them seriously than not.
Edit: typed all that, scrolled down, some other dude already said it
tjhart85