[RPG] the best way to plan out traps in dungeons?

I have experience playing the low levels. I can briefly summarise the impact as follows:

• It will make encounters much harder. With many characters dying in combat, and possibly a few total party kills as well.
• This can be demoralising, but some players might be up for it.
• But something perhaps easily overlooked is that it removes a wonderful suspense building mechanic from gameplay. With current rules, a character going down leads to a change in tactics to keep the party member alive. By removing death saves, this falls away. And the only decision is: Is it time to flee, or can we still win this?

Characters in our party are regularly taken down (which would mean death with the house rule) and require in-combat healing to bring them back into the fight. Failure to do so in some cases would probably have resulted in total party kills.

In your comment on Dale's answer you consider:

players were encouraged to retreat if the going got tough

The problem with this is that there's not necessarily enough time to make an escape. The fact is that even as it stands (without the house rule): "level appropriate" monsters have a decent chance of 1-shot killing most level 1 characters. (When I say decent chance, I'm not even talking about needing a critical hit.)

NOTE That a single CR 1 monster is considered level-appropriate for 4 level 1 characters.

Consider a Specter (CR 1): It does 3d6 damage with +4 to hit. This is only an average of 10.5 damage. But it's special ability states that if a CON saving throw is failed, the target's maximum hit-points are reduced by the same amount (and if reduced to zero, the character is dead).

Of course if you and your players are up for a much more difficult challenge, then then feel free to tweak accordingly.

However, changing the death-saves rule is not the only way to do so. Remember that removing death-saves also removes a tension creation mechanism of combat. So instead of removing death saves, consider the following ideas:

• Have your monsters fight more "intelligently". Let them make good tactical decisions, and you'll see difficulty ramp up without any extra work.
• Tweak difficulty of encounters by adding monsters, or using stronger monsters. This might require a little more planning on your part, because it can be a little to easy to overdo it.
• Reduce the character's opportunities for rest a little. This means they rarely recover their abilities between encounters. It has a similar effect to the second point but is possibly a little easier to manage.

Your problem is meta, solve it meta

Tell other players, including your DM, that you are planning to play this character that does justice serial-killing. Other characters might feel this character is shady, which is fine, but ask them not to uncover your hobby too quickly. Tell them that you want to develop this character, at least for x sessions.

The in-universe explanation would be that most people tend not to stick their nose in other's business, and respect each other doing in their time. Ask for your DM's cooperation.

• If they try to follow you, DM will say they lose you after a while (you should be trained in shaking pursuers, I guess?)
• If they become suspicious of you, ask the player to explain why their character became suspicious and what they are suspicious about. Everyone has secrets to hide, and they won't know exactly what you are doing
• Start stating where you will be hunting your prey after others state their whereabout. You can choose victims where other PCs are not there
• Basically, whatever they want to do, you always escape and no one knows/confronts you until the time to do so, which is when you say so

DnD is not a game of hide and seek, a competition where other players want to uncover your dark side, unless you are playing the villain. Even so, DnD is a game of story. What fun it would be if the villain is uncovered on first session and killed? Instead, ask their help to build the story, until the day where you think you want to play another character, when their suspicion is proven and this time you fail to escape and must accept the judgement.