How come when some people commit suicide by hanging themselves their head swells & turns purple?

When you hang yourself- it can either snap your neck, or you can die from suffocation. When a human being suffocates, their skin often turns into all kinds of horrible shades of blue and purple.

When someone is hanged, their neck breaks or their air supply is cut off. And since the rope (or whatever they used) is so tight around the neck, their blood vessels in their neck and face expand and eventually explode, causing discoloration. Pretty gruesome sight