Is THIS what little boys are made of?

A small recap and a disturbing add-on:

So, last week, we went to my friend J's house to play. While I was inside with J. the boys stayed outside with J's son L. who's 8 and his friend, "T", also 8.

After awhile, Matthew came in looking distressed and worried and wanting to whisper a secret in my ear. I bent down and heard this:

"L's friend just said to me, 'If you open your mouth and say another word, I'll shoot you down dead."

Apparently, "Tom" was pointing a toy gun at Matthew's head while uttering this threat.

I mumbled something about  Matthew playing on the swings for a bit, assuring him that "Tom" didn't really mean to scare him and reminded Matthew that hurting someone is never acceptable, either.

But in my head/heart? I wanted to kick that kid's ass to the moon whilst ripping his face off.

Tuesday, I had L and G (little sister) over to play while J ran errands. Yesterday, out of nowhere, Matthew goes all quiet and contemplative. Gets that worried, anxious look on his face and refuses to engage while clearly working through something in his mind.

Finally, with tears streaming down his dirt-covered, popsicle-smeared little face, he confessed what he'd been thinking about:

"Yesterday when L was here, he said that I'd better beware because he's the Joker in real life and he's gonna kill Daddy."

Seriously? SERIOUSLY? What the HELL am I supposed to do with this? I wrapped Matthew in a huge hug and told him that L was very naughty to say such an awful and scary thing and that it's a lie. L can't hurt Daddy, EVER and if L ever says ANYTHING like that again, that Matthew should come directly to me or Julie and let a grown up set L straight.

I'm at a loss, otherwise. I LIKE J and until now, I've really liked L, who's been good with the boys. But this is not acceptable to me and I'm not sure how to approach it with J.

Thoughts?