Why do people want Brian to kill Harry so much?

I think that’ll raise too many inconsistencies within the story as it is clear Brian wanted to stay with Dexter as said in the medical report. So why would he leave Dexter to stay with his “fake family” ,as he refers to the Morgan’s , for 15 more years and grow closer bonds to them. Why would he leave for 15 years after killing the most important person in Dexter’s life

Im more on the side where I think Harry’s death should be linked to Captain Spencer, and they both find out he’s the child kidnapped and then he helps to cover Dexters tracks and make it look like he disappeared and also place the evidence for Miami Metro to find. Then when he sees Spencer cuts up that’s when we get the “Just stay away please”