Well, Ennis did have family. He had a brother and sis + two daughters that he seems to care for very much. I don't think if he lost Jack he'd just curl up and die.
Yes he loved Jack, wanted him, but he had a secret that he had to keep and was paranoid about keeping it. This limited his social life,seeing the more people he let in, the more risk of exposure.
The fact that Ennis wanted to be alone alot is not strange or suprising to me. Privacy is essential to him and I can relate. What's taken as lonliness for some is bliss for others. He worked on ranches, so no doubt had some friends and aquaintances. I feel even if he'd never met Jack the idea of crowds and people hovering around him constantly would not be his style of life.
After Jack, he may very well see romantic relationships as not worth all the hassle as many have done. But sometimes, this is when love finds people , when they,ve givin up and stopped looking for it. Then boom, the great love of your life comes along. Who says this can't happen in his case?
If it happens, great, if it does'nt, then he will continue to live life as he sees fit. If it's alone, so be it, if that what he wants. He loved Jack. No doubt about it, wanted to be with him but just could'nt be in the way Jack wanted him to be. ( living together ) If Jack moved on, I mean really moved on ( no contact, no meetings) with his life without Ennis, Ennis will survive. But only if he saw that there was no longer a chance of being with Jack again.
This is Jack's power over Ennis though it seems alot of times Ennis was in control of everything, it was Jack who really owned Ennis. It would take Jack moving on seriously with someone else to bring it home to Ennis, but as long as Jack is out there free and available in some way Ennis will always have hope of seeing him again. He loved him that much. After all he was a very handsome guy , no doubt able to attract people of both genders. Though a loner, Ennis was a diamond in the rough with much to offer. Jack saw it. I don't discount him completely.
Ennis never would have killed Jack. Felt like killing him at times?, as we all do at times ,lovers, husbands wives etc... , but to actually carry through on this threat of harming Jack in anyway was not going to happen IMO . He was very upset and jealous of the thought of Jack with someone else, but to physically hurt Jack? I don't think so, but he was human with human frailties. My opinion.