The instant kill block will only kill Mario if he comes in contact with it, it won't 'randomly' kill him. There should be something else in the level which is causing that but with the lack of details you've provided it's hard to tell.
Maybe the block is set to kill Mario when sprites come in contact? That's the only reason I can think of. Post the code of that block here. If it's a .bin block, make sure you set the sprite offsets to -1 (or whatever number disables it, I don't exactly remember).