Get outta DC sweetie. Yes, you could have followed him to McDonalds and shot him down. If the cops arrive you have to stop and let them take over.
You really need a reality check on Texas legalities, and how they are shaped by the culture. East coasters need not apply.
Ludicrous.
Give me a case that backs up your assertion that a citizen can execute Kelley if he is sitting at a table at Mcdonalds calmly eating a hamburger 20 minutes after he killed 50 people.