My children are 14, 11, and 7. I answered a similar question from a mom whose 4 yo is using bad language.
First, the reasons for this could be many, his age, his personality, his reaction to his sibling who is now developing a personality of her own (and getting attention of her own that doesn't revolve around simply taking care of her needs).
My suggestions to you is to remain calm. This is the most difficult part of the whole thing (it is really hard not to react), but if you come unraveled every time he acts out, then you are both in the same boat. Someone has to be in control, especially when the other person is out of control.
Next, let your action fit the behavior. If he is using bad words, simply say that you cannot hear ANYTHING that he says when he uses words like that. He will have to repeat what he is saying, minus the bad words, if he would like you to hear and respond. Same goes for whining. If he is throwing a fit, let him know that you'd be happy to talk to him, once he pulls himself together, and walk away, if you are at home.
Try to talk with him about what is going on at a time when he is not having a meltdown. Perhaps he is experiencing an issue with a child at the daycare, or perhaps something is bothering him that is completely unrelated.
You need to find out if he is so angry with someone that he actually would like to hurt them (I'll shoot you), or did he just say it because he thought it sounded tough or cool. In either event, you have to help him understand that threats like this are completely unacceptable. Most schools have zero-tolerance policies for threats of violence, even from the youngest students (who usually don't mean it). It's worth helping him learn about it now, than having him suspended over it later.