Because the walk animation and the fire animation are true at the same time then both animations are trying to play. So you must block out one of them with a condition.
So on the walk events you must add a condition like 'is not firing'. In the case of your game this could be one of the variables since you use them for firing. You could possibly also use 'rifle shoot animation is not playing' as a condition, it depends on the logic you set up.
The key is to not allow the events to be true at the same time. If you are firing then the walking animation should not play, hence the walking animation event should be 'play walking animation when pressing WASD if you are not currently firing'.