That's the point. He did basically what you said he should do. It's like you're saying, "I agree with his decisions, I just don't agree with the outcomes they led too." I don't have any information on when precisely Stalin decided he wanted Trotsky dead, but you can't just assume that he can write his name in a death note and have him drop immediately, and if one guy surviving ten years in a foreign country can destroy an entire revolutionary project just by writing then tbh it seems like the whole thing was doomed from the start. You can't predict every such case, it's not a valid criticism.