Why is Osborne a suspect in Magnus' murder in the novel, Magpie Murders?

Osborne became a suspect because he was gone away from home at the time when Magnus died. He told his wife that he had to go to church to pray and try to calm himself down. Robert had seen Osborne’s bicycle outside the church and had taken it to go to Pye Hall. Osborne, who had been at the church the whole time, had no idea his bike had ever been taken. When he rode the bike home, however, Osborne got some blood on his sleeve, presumably from where Robert had killed Magnus.