We had issues with our dog biting when he was a pup, but it seemed to get better after neutering. Now after using Frontline he is acting very aggressive and growling & snapping at us. We're scared. Euthanize?

We got a black lab/spaniel mix 10 wks old, although the vet thinks lab/beagle. Puppy biting was pretty extreme. I had bite marks for 3 months--put him in crate for time out when it was really bad. I know puppies bite but he actually went upside down on the floor & snarled. It stopped after 3 mos. Had him spayed at 7 mo. Amazingly intelligent-learns in 1 or 2 sessions, knows 30 different things including picking up toys by name. Follows me everywhere, NEVER away-which I love. 10 mos old now. Vet put him on Frontline Tri-Tec. A.M. after 1st dose he wouldn't get out of cage for 1 hour, acted like he didn't know us, then was ok. 5 weeks later-2nd dose-5 DAYS later he snapped at me when I put him in his crate for bed. I went in 2 hours later, he gave a long deep growl. In a.m., would not get out of cage, growling at both me & my husb, snarling, lunging, trying to attack thru the cage like a street dog. Vet heard him over phone, prescribed 2 tranq. So we could bring him in. Took 3 hrs to work. We're scared. Euthanize?