The DA does not have to prove you killed the guy.That is a given.The burden of proof is on the shooter,to prove self defense.
I don't know about other states, but in Texas the burden is on the DA.
Texas Penal Code Section 203(d) says, "If the issue of the existence of a defense is submitted to the jury, the court shall charge that a reasonable doubt on the issue requires that the defendant be acquitted."
And self defense is a defense - as opposed to an affirmative defense which works differently.
As far as the positive results of a drug test go (though I don't know why you would be taking one), it's an indication that you MIGHT have been on drugs. I imagine that the amount of whatever they are testing for that is in your system would be an indication of how buzzed you were at the time of the test. Which might not be the time of the incident. So it doesn't seem like really strong evidence.
At least in a DUI, the test is done right away.