Is it "crazy" to vent anger at an ex via text msg?

No, it's not wrong to vent through a text message. You were obligated to get rid of all the feelings that you felt and to clear your mind. This will indeed help you when it comes to getting over the breakup. However, I do believe that i would have been better to do it face to face. Thus, he knows your true emotions and not the silly words written in a text.