It's not hate really though is it? It's exasperation born out of frustration I think (at least for me, anyway). Yes, he works hard but not smart. He charges around the pitch like a bull whilst his brain is often in neutral and gives away countless fouls. It's sometimes akin to watching young children play football where they all chase the ball like wasps with no real idea of where they should be at any given moment.
He scores goals sometimes, this is correct but only really one dimensionally. Defenders know that if they can nullify head height crosses, Slimani probably won't offer anything else. There could be a very good footballer there somewhere and with time we may see it but all evidence thus far suggests that he isn't a very bright one.
It's not a case of expecting the world for £35m but if we'd seen him play last season for a different Premier League club, like the way he has played for us so far, there would be apocalyptic outrage if we were offering anything more than about £10m never mind 30 odd.