I was just wondering if it is on purpose that reapers can kill people through doors or if this is a bug. And if it is intended could someone give the reason for this
Probably not a bug - the sickle functions similarly to the harpoon in that the entity is ahead of the character model enough to pass through doors and some walls.
Yup. I checked again and it is indeed intentional. It was done to stop people from hiding in places where the reapers cannot reach them at all - causes delays.