Well, no it's not "justifiable homicide." It's just plain old criminal homicide. If the guy really did do it, I'm not shedding any tears over him. But Ms. Cunningham (nor anybody else) is allowed to simply kill those whom they think "need killin'".
Well, that may be what the law says, but in many parts of Texas and probably elsewhere across the country a case like this might very well get no-billed. If it happened to my daughter, I'd respond the same way. Expecting "justice" from our legal system is a futile effort.