Well i think they only killed lenny, sean, hosea etc. because they would have been on arthur and johns side at the end. the whole thing was set up so it would be arthur and john vs. the remaining van der linde gang members in a final fight

I liked Lenny and his charisma but I was more gutted when Hosea died. It really felt like he held the gang together and that when he died, the gang lost its way. He didn't deserve to be gunned down in the street like a dog.