pump full of lead
To overkill someone with a gun. To shoot way more than the required amount to kill.
I pumped him full of lead for not paying up.
Pumped full of lead
To be killed with any type of firearm (except a hunting rifle), with multiple bullets to the body.
Since bullets are made of lead, the phrase pumped full of lead makes sense no? And besides, learn what it means snitches get stiches the easy way, I.e not by being pumped full of lead.