By putting the older dog down, i think you did the right thing. You are saying thats not ok, and now you get no attention. As sadie has gotten along with other dogs, it was a different story when i brought the new pup in. Because now the pup was in her territory shareing her treats or mom . Sadie did not want the pup to sit with her, lay with her, eat food from THE PUPPYS BOWL. She became aggressive and possessive. Every time that i saw this behavior happen, i would correct her. Took about 2 weeks everyday and every time it would happen. Because sadey wasnt used to a pup shareing her space. Though she still corrects the pup, if the pup annoys her while shes sleeping or is annoying her period. I let her correct her as long it doesnt get dangerous. She also get jealous of the new pup and when she does i simply get up and no one gets my attention. sadey is spayed and i plan on spaying bella when she reaches 6 months. So hopefull it will work out.