Death is a known possible outcome in most SWATers' minds, and they often make jokes to that effect, or jokes about killing people's dogs (and dogs have indeed been killed due to SWATings). They'll often make false claims like "someone is being held at gunpoint, it looks like an automatic weapon". They're knowingly gambling with people's lives when they claim there's an active hostage situation in someone's house. This is not just a prank or hoax.

The SWATer absolutely shares responsibility for their death, even if they don't hold sole responsibility. 20 years seems like an appropriate sentence to me.