I really wish that RDJ had used a stuntman. He is the lead actor and the last thing this movie needs is for him to break a leg and delay this entire movie for several months. That's what the stuntmen are there for, doing things that might be dangerous.
Now that this has happened, hopefully they'll be much more reluctant to let him to risk himself like that.
He probably thought it would be more exciting if the audience saw him doing the stunt.
Lots of actors want to do their own stunts(tom cruise)
This picture is not where he sustained the injury but I believe RDJ was doing this scene himself