Law wants to kill yonko, so does Luffy. Law wants to do some some SAD Crap too. Law has no time to do so much.
- Bing* I know I'll ally that fool
Luffy is gullible and decievable. Law can just get some help from him and his crew and kill Kaido. Then he traps them in that room thing and he will chop them up into tiny pieces and eat them.
Law is yonko and he killed SH Crew...pwetty boos.
Then he becomes intimate with big mom and together they kill the yonko and become pirate king and queen.
As so is the circle of life.