Mouse died on glue trap, why?

For the past year we have been dealing with the burden of mice. We live in an older home so we kinda figured there were some pests. The previous owner had left some snap traps so we set those up and never caught anything, which was kind of relieving because I feel bad just killing them for trying to survive. Well we moved to sticky tape traps, and those sneaky things got out every time, there would be poop and hair on them but no mice. So then we moved to glue traps, I had watched videos on removing them from the trap so we could relocate them. Well we set 7 up about 2 weeks ago and I checked them every morning and night. Well I just came home from work and there was one stuck in the trap, and it was dead. I feel so freakin bad. I don't understand how it died. Are there any traps that are actual traps? Not little rodent killers. I haven't been able to find any humane traps or at least not any for a reasonable price.