I didn't understand Katniss' "mission" in Mockingjay [SPOILERS] [DISCUSSION]

Why did she risked her and her team's life trying to reach Snow? It's so clear that the rebels were winning and slowly and steadly reaching Snow's Mansion. That was clear BEFORE Katniss decided to hunt Snow and also WHILE she was on her way.

Did she fear she would lose the opportunity to kill Snow herself? If that's the reason, it's so selfish of herself. Even more, President Coin already promised she would be the one who will execute Snow.

All that ruined that part of the book for me, it seems so pointless. Maybe there is there something I missed? Thoughts?