Definitely sharpened teeth coated in fast-acting poison, but...
Bear traps are not designed for lethality, they are designed to minimize damage done to animal's fur so it can be sold for the best price. Ill provide ways to make bear trap more lethal and examples of other traps that are more lethal by design.
Upgrading bear trap
- Camouflage it. Catching someone in a trap is the most difficult part. Paint trap according to environment and add bits of foliage or ground from around, leafs, moss etc. Be careful not to leave tracks.
- Sharp teeth and poison. This way trap will be more lethal, most of bear traps use rather blunt jaws to break and hold prey's leg, not very lethal. Sharp teeth are also way better at delivering poison.
- Not necessarily stronger spring. You don't want to cut off prey's leg, sure they can bleed out eventually, but it decreases chance of delivering poison into the blood stream and if they are intelligent they'll most probably escape, without a leg but alive. Perfect spring would allow trap's teeth to penetrate deep into leg's flesh and break bone, but not cut it off. Depends how big differences are between orcs, trolls, etc. You might need different springs for different types of prey.
- Eventually make it bigger. Not the most efficient upgrade, but by increasing diameter of jaws you can change the location of wounds, assuming prey is of humanoid shape thighs and abdomen are prime targets. Of course this trap will be much heavier, harder to transport and hide.
- Pitfalls with spikes. Very lethal, not very portable.
- Swinging log traps. It will kill anything with big enough log.
- Anti personnel mines. Portable, lethal and used at least since 18th century. Although chinese allegedly used them over 600 years ago
- Most of traps used by vietkong during vietnam war.
- My variation of "bamboo whip". It can be dug in like in the picture or mounted on trees, pretty easy to manufacture, transport and hide in thick foliage, not as easy to set up as bear trap, but designed for killing. Although take into consideration need for some fantasy material to make the rods, even bamboo is not that flexible!
P.S. Traps made from surrounding materials are theoretically better because: they already blend-in with surroundings, their smell isn't out of place (it might be important for hunting wargs) and you only need to have knife or hatchet to make them, eventually some rope, instead of transporting whole trap.