Hi Sparks,
I can see that you loved your dog very much, and feel bad for one other as well. It is possiible that the man in question killed the dogs. But, like others have said, to prove it is most unlikely. I understand you feel angry. The man you describe may well be a creep. But, when it comes down to it, the only thing you have a 100% chance of changing--is you, and the actions you take to be more cautious in future situations.
It may have been a tragic one time event where you let both dogs out for only five minutes. Unfortunately, sometimes that is all it takes. Dogs often wander much further than their owners realise--especially if they are in, "pack mode." The two dogs were together that day. There may have been nothing left of interest in your yard, and they may have gone into exploring or hunting mode. They were not shot on your property, right?
I've known some people who have let their dogs run for years with no problems. And, I've known some who have had several in a row go permanently missing, shot, snared, or poisoned due to some threat from the dogs to others or their animals, REAL or PERCEIVED. Additionally, one dog can run loose every day for 15 years and never get hit by a car. The next two dogs in a row can be hit after one week of running loose. In other words, there is substantial risk for dogs that run loose to be killed in one way or another. Period. A dog, or cat, is no match for a car, gun, poison, pack of coyotes, loose pack of dogs, etc.. Best to you in what you decide to do.
BTW, I have German Shepherds--and they have made it clear to me that they will remain perpetual lap dogs.
Lara