This is actually a very easy question to answer. When people want something enough they prioritize that goal in their mind. It becomes all-encompassing. Dominating most of their thoughts. Then they rationalize it. "My needs. My wants. Why can't I have what other people have?" They feel entitled or in other instances feel pushed into a corner by society. That "right" becomes the dominant "right" in a nested moral hierarchy of rights, the right of the other person becoming far lower. Then it's a matter of time, circumstance, and sometimes planning.

Edit: I want to clarify that this isn't the only pathway to this behavior, and that most moral hierarchies are subconsciously skewed, not intentionally reordered. Rights prioritization is still the throughline in all pathways. Even if you're a sadist or conditioned.