If you're not talking about "mature" in the ESRB sense (and thus i assume the general "not appropriate for kid" definition), then i don't think DR3 is a particularly good example :p In this context, having blood and violence no more makes something "mature" than a lack of it makes a game childish. DR3 uses both as a counter point to its own childishness, which adds a lot to the humor.
As for hardcore, the word itself usually just refers to the most active or committed members of a group, but there's no universal definition among gamers. Most people apply a lot of conditions to what can and can't justify its use, which is fine, but it results in the term being rather open ended. I personally follow the traditional meaning. If you game frequently and are committed to it as a hobby (not just a way to kill some time), then you're "hardcore" in my eyes.
