There are degrees of murder depending upon both the evidence availible and the severity of the crime. As for your examples...
1. Ants are not humans, therefor human law cannot be applied.
2. It depends on the circumstance in which the criminal in question was killed. Was it self defense or defense of others? Was the criminal killed by the person whom he was robbing? Was he killed as a result of his criminal activities or by a premeditated act? Is there evidence that it was a premeditated act?
3. The murder of a president or any other head of state is almost always a premeditated act. The sheer amount of protection details that surround a head of state make an event like the accidental death or killing in self defense of a president statictically unlikely. Also most assassins or would be assassins are killed on the spot because the secret service is trained to shoot to kill.
The point is that the sentence is not determined by the "worth" of the person murdered but rather the circumstances surrounding the murder, the evidence pointing to the suspect, the suspects state of mind, and numerous other variables that you did not even consider.
Infinite punishment for finite crimes is inherently immoral. Case in point, there are still a few native tribes in remote areas that have never been in contact with christian missionaries, or anyone else for that matter, do those people deserve infinite punishment for their ignorance? Infinite punishment for no other reason than that your "god" chose not to reveal hinself to them?
Here are some other examples to put things in perspective.
Is simply not beliving something that you cannot be convinced of worthy of infinite punishment?
Your religion also demands infinite punishment for crimes far less severe than murder, is picking up sticks on the sabbath worthy of infinite punishment?
What about not hating your family, which is what jesus said was required to truly love him?
What about all of the other relatively minor offenses that your religion demands infinite punishment for?
What about a little girl who doesn't respect the father who raped and beat her, does she deserve infinite punishment too?
Can you see the flaw in your thinking now? Can you see that infinite punishment for any finite crime is immoral. Period.