Why is death horrible for those who have not experienced identification with Christ?
Because without identification with Christ's death and resurrection, there is no blessed hope. Death does not lead them to something better; it leads to something a thousand, a million times worse. Lacking that union, they face death with fear rather than the saint's glorious prospect.
Les explains that believers who have been planted together in the likeness of His death can believe Christ died their death, and so they shall also be with Him in resurrection. That is the blessed hope: they will not live and die like a dog or face a prospect of eternal doom. For the saint, death is glorious, just on to something far better. But for those who have not experienced this identification, death is indeed to be feared and horrible to experience, because it is not going to something better; it is going to something a thousand, a million times worse.