When death comes (line 1)
The approach of death is, well, frightening. We bet that for every movie you can name that doesn't have the threat of death as the main source of tension and fear, we can name ten that do. What we're trying to say is that fear of death is pretty universal. Living things tend to have an aversion to death right? That's part of how we stay alive. Still, it's worth noting that this line does not try and crank up our fear to the boiling point. It's a fairly calm and contemplative entrance to the idea of the moment of death. But it definitely undoes the latch and lets fear into the room.
like the hungry bear in autumn (line 2)
OK, we don't want any kind of bear to come at us, much less a hungry bear trying to stock up before winter. More than anything else this image suggests the implacability of death. Bears are big. There's not much you can do to stop a bear that's coming at you. It's a way for our speaker to say: death is coming, and it's not going to be stopped.
like the measle-pox (line 6)
If we thought things couldn't get any scarier, now we have all the suffering of a long drawn out disease to consider. Our speaker is giving us a variety of images and comparisons. Death is a complicated thing, it seems, capable of coming in many ways.