Kill you? Come on, they're not going to add an additive to any product that will "kill you". I think the clinical trials required before it gets released on the general public would have noticed that side effect!
Whey protein shakes are a good source of protein if you find you can't get enough from food choices. Food is always better. If you don't feel like meat, look into high protein other food choices such as beans and pulses. But if you still can't meet your goals, whey powder is one way to help.
Aspartame is harmless in any level that you're actually going to take it in. Just don't have 346 shakes a day. :)