I find that PC death is usually due to poor thinking on the players part. I run lethal encounters all the time, but as a DM, I feel its my duty to leave at least two opt out options for the players, one always being RUN !!!
One can say that most combat encounters are to be lethal. Would a marauding band of Orcs choose first to capture the PC's or kill them outright. Understood, if the Orcs have a background agenda, capture might be in the mix, but, on average, the Orcs would seek to kill off the party, you know, with Humans, and Elves, and all that. That is based on hate alone, in most cases.
In my experience, alot of how the players handle situations is based on DM to player talks. I try to hold these talks about every 4th gaming session. What I do, is, I go back through my journals to past games, and discuss how those past situations could have been handled differently. I offer my advice, and suggestions. We discuss how THEY, the players, would handle this and that if THEY were actually in that situation, ie; in combat, when the party tank gets laid VERY low on HP's, especially in one or two hits, the cleric has about 3 healing spells left, the mage is half dead, and cooked on spells, and the rogue can stub his toe and die, and the BBEG is still too perky, and pissed off, well, it may be time to get outta dodge, and re-try the encounter on different ground, on a different day, etc.
In the case of a Beholder: the party should fear for their lives: PERIOD. They are a most fearsome beast, and should be respected.
Too often I see 'the CR is THIS, and this is so and so...'. Well, I dont adhere to the CR and XP system in 3E to a flaw. I dont want it where a player can tell ME when they get enough XP's to go up a level. I will many times insert a CR 10 creature where the party would be maxed out on a CR 6. Why? Who's to say that an adventuring band, while making their way through a mountain pass, wouldn't accidentally stumble upon a Red Dragon's lair, or at least an auxillary entrance to one. The party is cold, windblown, tired, and viola! A cave .....Unless the region was WELL researched by the PC's, or they had good opportunity to have heard about the area from say a Bard/Sage, well, then they are travelling blind. Just like travelling in any large city. Unless you KNOW where you are going, either by research, or by a friend's advice, you may not know where its most likely to get mugged...
But, when you ARE travelling in a city, and see the surroundings change to a more 'seedy' appearance, you might want to backtrack out of that area. You have the opt out.
I also on occasion make an encounter based on ONE players abilities/skills/feats. If I have a player who is sidelining alot, this is my way to bring them out.
I suggest talking to your group, as has been suggested. Find out what they want to get out of the game. Ask them DIRECTLY how they might have better handled PAST encounters. Then offer forth your idea about how the game should go, and COMPROMISE. Mix it up between YOUR game, and theirs. If they are concerned about YOUR choice of targets for spell effects, so be it. if you have 6 players, openly roll a d6 in front of the screen, then nod your head at each player as you count around to the target player. This will let them know that you are being random. After a few sessions of those open rolls, you'll find that you can again move those rolls behind the screen. Then, just make sure you roll a die behind the screen before you unleash, even if you KNOW that you'll be targeting so and so, make the random roll and nod out the count. So this way, if player 4 is 'Steve', and you want to make Steve sweat with a little green ray, he'll think its random.
BUT, for intelligent monsters, it's your duty as a DM to play it all out. A Beholder is smarter than most PC's. They live a long time. They have been in combat before, most likely, and fully know their strengths and weaknesses. As long as the PC's have a way to run away/hide/regroup, then go for it.
Orcs hate humans, elves, and dwarves. To them, the only good dwarf/elf is a dead dwarf/elf. And look at all that meat
Have that talk with them, and TAKE NOTES on their feedback. That way, you look concerned about their feelings. Take a week to study their wants/needs, and retool the game with a COMPROMISE. But let them know that death can be and IS a potential part of the game.