It sounds like black humour to me. Making a joke about a serious situation. If you get a minor injury you can 'walk it off'. It's an expression you hear a lot in sport. For example a man might bang his knee playing football. It hurts initially, but you can 'walk it off' i.e. you can carry on as normal and within a few minutes and without medical attention the pain will go away. However if you are killed, there is no way you can walk it off, because you are dead! Someone is making a joke about a serious situation. Without more context it is hard to know about what exactly. It could be literal, or it could be a metaphor for something else. If taken literally I could imagine an army captain saying this to his soldiers before a battle. Getting killed is a serious consequence, but if you make a joke about it maybe they will feel more comfortable about the prospect of the battle. There are other possible interpretations of this phrase, without knowing where you found it I cannot say for sure.

It needs more description of the situation to give an accurate interpretation. Here are some alternative uses. It used by one friend/colleague to another person. They are trying to comfort the other person who has a stressful situation coming up for which they have prepared for e.g. a performance, a presentation, a competition in sports or even a video game. "even if you get killed..." means if the result was the person did not do well as they wanted or even badly. For example, a student gave a presentation and afterwards the lecturer asked questions from the floor. But the student could not answer most of the questions. That means the student "got killed". If in sports, for example, a table tennis match, player A is against player B. Player A is trying hard and wants to do well but player B is known to be a better player. So, a friend might to player A "even if you get killed..." which means "even if you get heavily defeated....". In both scenarios, the student and person A might feel a lot of disappointment so the friend says "walk it off." This means, go for a walk (or do something else) to stop thinking of the disappointment.