It does not matter what his plan was when he entered the bar – he could have gone there just to get a drink, and then formulated a plan to commit murder once he was there. Ideally, if he actually premeditated the act, then the evidence should show what he premeditated the act, and if he did not, it would not. The general pattern is that the prosecutor accuses a person of doing something, and the evidence that is presented is tightly related to what actually happened.
The way this is sorted out is that the jury is given an instructions that describes what the law means when it talks of "premeditation", and the jury has to device if this situation has been proven. My example is drawn from California (Kansas instructions are a struggle for me at the moment):
The defendant is guilty of first degree murder if the People have
proved that (he/she) acted willfully, deliberately, and with
premeditation. The defendant acted willfully if (he/she) intended to
kill. The defendant acted deliberately if (he/she) carefully weighed
the considerations for and against (his/her) choice and, knowing the
consequences, decided to kill. The defendant acted with premeditation
if (he/she) decided to kill before committing the act that caused
death. The length of time the person spends considering whether to
kill does not alone determine whether the killing is deliberate and
premeditated. The amount of time required for deliberation and
premeditation may vary from person to person and according to the
circumstances. A decision to kill made rashly, impulsively, or without
careful consideration is not deliberate and premeditated. On the other
hand, a cold, calculated decision to kill can be reached quickly. The
test is the extent of the reflection, not the length of time.
The prosecution is required to prove that there was premeditation, which means that they would have to present evidence, perhaps in the form of statements made days or minutes before the act. It's possible that a person could all of the sudden lash out against another and unthinkingly shoot him, perhaps planning to hurt but not kill. Other statements or actions could support the claim that the killing was not spur of the moment.
As far as I can tell, the case could not be dismissed (it is not in question that he did the act, that it was not self defense, and there are no apparent procedural screwups that would preclude inculpatory evidence from being introduced), but there could be a diminished capacity defense.
In other words, if he did premediatate the act, and if convincing evidence of that is provided, he may be convicted. If he did not premeditate the act but is still convicted (of 1st degree murder), then that would mean that the evidence of premeditation was misinterpreted – the outcome that is not supposed to happen. And if the jury does not find that there was premeditation, because the jury finds the evidence to not be sufficiently convincing, then there will be no first-degree murder conviction.