I think it's immoral to kill bugs if they're not egregiously bothering you. Even though they are less sentient than us perhaps they still have some basic will to live and avoidance to dying. It doesn't seem right to just step on some ants or flies or something. I walk around them or if they're in my house I leave them alone or shove them outside. If you we're talking about bacteria I wouldn't mind. Their life is so lowly it can hardly be considered immoral to kill them capriciously. I like to kill a lot whenever I sterilize something.