Murder is a tough thing to digest, it's a slow process
And I ain't got nothing but time
For many people, it takes a long time to come to terms with murder. By saying he has plenty of time, he tells you he has no issue with killing someone.
This is also a demonstration of his ambition. By suggesting “i aint got nothin but time” he envisions himself being around for a long time, even before he began.
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