nothing wrong atall mate, people have differing opinions about season's and dispatch methods for quarry, shooting hares with an air rifle is fine and pheasant and partrige and duck (in season, with permision).
No closed season on hares mate, however it is better to let them be during summer to preserve sport as they arn't really a pest as sutch and so they can care for young ( so there are more for you to go at in future ).
Some of the people who tell you that you shouldent probably think nothing of taking as many as they can during winter.
I dont see the problem your first kill a