You are absolutely within your rights. Legally and morally. If some random asshole dogs came into my yard and started killing my babies I'd kill 'em, no questions asked. You probably love those guineas just as much as the owner loves those dogs (if not more). It's not ok at all for them to come onto your property and kill your animals. That's insane to say that murdering your pets was fine, but it's not ok for them to be killed for it, along with the trespassing. I LOVE dogs, perhaps to an unhealthy level, but any animal, just like any person, has the potential to be an asshole, and I wouldn't hesitate to save my sweet babies from whatever was trying to hurt them. I am their momma, and I would die to save them (yes, I am an insane pet person to die for a chicken or a hamster, but that's who I am). Even if it was two of my own pets going against each other, I'd kill the aggressor to save the other if I had to. It would destroy me, but I'd do it.
And if my precious little dog trespassed into someone elses yard and murdered their pets and was killed for it, obviously I'd be horribly sad, but I wouldn't fault the person defending their pets. If my dog was in the wrong it'll have to face the consequences. It's on me for being a negligent owner and not taking better care of my dog/keeping it out of situations where it can get itself killed for following its less pleasant instincts.
I'd definitely post the law on your neighborhood forum and alert the authorities at this point, just because people are gonna be gunning for you now. But in the future, I agree with the poster who said to just kill them and dispose of the bodies without anybody knowing. Can't get sued if they just disappear and nobody ever knows what happened to them. Take the collars off. Dispose of them in a public trash can somewhere, and be done with it. The less law enforcement/lawyers/litigious people involved the better.