Try adding warnings and incremental degrees of deadliness to your environmental hazards.##

Instead of a single failure leading to instant death, give your players chances to realize the danger and deal with it. Giving your players more chances and information makes them responsible for their potential deaths instead of you.

In the example of the rickety bridge, maybe it's stable enough that nothing really happens when the 35 pound Halfling Monk runs across. When the unarmored Human Wizard crosses, maybe there's some creaking and shaking, but no checks required. But when the 250 pound Goliath Fighter in full plate mail takes that first step onto the bridge, let him know that the thing starts creaking and trembling. If he continues, have him make a series of checks (probably Acrobatics) to see if he can step gently enough to keep the bridge stable. Rather than a single failure leading to death, however, maybe the first failure simply causes a single plank to fall from the bridge. A second causes a whole to break open under the fighter, but he is able to catch the edge and pull himself up without making another check. Hopefully at this point, he'll realize that he should find another way across. Perhaps an ally could cast Levitate or Fly on him, place him on a Tenser's Floating Disk as they cross, or simply use Dimension Door if they're high enough level. Maybe he's going to have to take off his heavy armor to safely cross. But if he's reckless enough to continue on at that point and fails a third check, that's when he makes the plunge. By making sure your players are absolutely aware of the deadliness of what they're attempting through warnings and the obviously increasing danger of each failed check, you also give them the burden of responsibility. It's not very smart for a fully armored Goliath to try to cross a rickety bridge, but if the player makes that decision and you give them a few chances, that's on them and not you.

Now let's take a look at the Materia factory example. Let's say, for example, that casting spells that deal Lightning damage is the trigger that could potentially set off a chain reaction and blow the place sky-high. If the party Wizard casts Lightning Bolt or Witch Bolt on turn 1, don't just say: "the factory blows up, game over". Instead, maybe describe that the materials on the nearby conveyer belt begin to glow and spark, and the party can feel the ground shaking around them as the energy spreads to the rest of the factory. If they don't get the hint, the next time a Lightning spell is cast, maybe one of the nearby Materia weapons explodes, forcing one or more party members to make a Dex save or take some damage. On the third Lightning spell, several such explosions occur around the room, and they can be heard elsewhere in the factory. At this point, it should probably be pretty clear that 1: Lighting is triggering the reaction, and 2: the reactions are getting more volatile with each occurrence. With this information, the party should hopefully see that now's the time to stop casting Lightning spells.

tl;dr: As long as the party has enough information, making a suicidal decision is their responsibility.