Someone that would enter a church with a test tube of tainted blood as you describe is trying to make a statement and wants to draw attention to you and your beliefs. Pulling a gun in such a situation could have far more devastating consequences than the tainted blood.
The weapons of our warfare are not carnal, but mighty through God to the pulling down of strongholds. 2 Cor. 10.4
Lest Satan should get an advantage of us: for we are not ignorant of his devices. 2 Cor. 2.11