Not sure why this is being debated. Other people (including yourself) could bear part of the blame, but ultimately the mugger decided to do the mugging.
Is this valid too if the mugger's family hasn't eaten in 3 days? Because it's easy to talk from a cushy home, with a full belly and a trouble-free upbringing.
One can feel sympathy for someone without absolving them. If they're so desperate, then why deny them their moral agency as well?
They are still to blame for the mugging, even if the reasons are understandable.