Free medical treatment can imply free drugs to a tweeker. Oxycodone for starters. Enter, fire gun, look for drugs, move along. A free clinic run by a local charity seems an easier target than your typical pharmacy. If they hadn't been jonesing for a hit so bad they might have waited and tried to burgle the place at night. That's my guess anyway.

Moths to the flame. We're all just bb's in a box, the more bb's, the more collisions. There are a helluva lot of folks out there affected by drugs that cause totally irrational (or reduce them to totally reptilian brain) actions.