Do hostage takers usually murder their only leverage as soon as a cop car pulls up? Certainly if someone has broken into my house trying to murder me, I'd be thrilled with two armed regular old officers showing up at my door to investigate the situation.

I'm sure there are situations where a swat team is genuinely needed, but I'm also sure those cases make up an extremely small percentage of the time they're actually used. In any case, if police are called out to an innocent person's house and they kill the innocent people there it shows pure incompetence on the part of the officers involved. A little restraint and a lot more training would probably go a long way to preventing tragedy.