How can I suggest the DM stop trying to kill us?

We have been playing a game for a while now, and while the game itself is quite enjoyable, the only problem is that the DM seems to be aiming to kill us. Sort of.

For example, there have been three situations where a player has been caught in a fight that there was no way they could manage:

  1. One PC (a Rogue with incredibly low AC and HP) was targeted by an enemy
    forcing everyone to leave the room, and locking them out, then
    grappling the PC and slowly draining the life from him (dealing HP
    and CON damage).
  2. Another PC was forcibly injected with Bugbear blood (being an elf, started to turn)*, and the time between being infected and finding a cure effectively turned him into one. It got to the point where the player was still in control of the character, but he was attempting to kill the rest of the party. We did finally manage to restrain him and force the cure on him.
  3. A third PC, while having decided to go ahead of the party, was attacked by a mimic. This player had worse AC and HP than the Rogue, and was quickly downed. In the same situation, my character nearly fell to his death (though I believe that was due to a misunderstanding, rather than the intent of the DM himself).

We all agreed (while not directly to the DM) that the third situation was incredibly harsh. There was no way the player could deal with that situation on his own, and it would have been better if he had removed the monster completely, or moved it to a situation where more players could have been involved.

The DM is not inexperienced (he has been DMing for almost 30 years), so we believe that this is his aim. 90% of the time, even if we get good rolls, we all end up a lot more worse for wear, and the other 10%, one or more of us is incapacitated, and needs to be revived. It’s starting to have an effect on the group.

How can we talk to the DM, and what might we suggest in hopes of changing his attitude toward “killing” player characters?

*Turns out this is plot device devised by our DM alone, I don’t think there’s anything in the rules about this.