How do take care of a assassin bug?

I can tell you how to take care of Platymeris biguttatus, the Twin-spotted Assasin bug. P. rhadamantus, the Red-spotted assassin bug requires similar care.
They can be kept in groups, they live together without problems and if you feed them enough, won't prey on each other. I often see my assassin bugs sharing as big roach, even with nymphs.

I would recommend a 10 or 20 gallon terrarium or tank for a group of 20 bugs, with cork or bark pieces as hiding places. They can be kept at room temperature and require no special humidity. I use potting soil as substrate, but they are just as happy with sand, coconut fibre bedding or tissue papers. Some sort of backwall (cork for example) will give them more room to climb. If you use a tank, a tight-fitting lid is a must. The bugs don't like to fly, but they can climb glass without problems and the nymphs are tiny.
Since they will leave plants alone, you can decorate the tank with such houseplants as pothos ivy, spider plant, bamboo ect. I mist the tank once every week and keep the substrate moist, for the plants and for the eggs.

I feed my group every week and they get roughly one insect per adult assassin bug and some additional roach nymphs for the babies. They prey on almost anything that moves: crickets, locusts, mealworms, butterworms, but roaches are my favourite because they are easy to breed. You will need a steady supply of insects for a assassin bug colony, so breeding your own is probably a good idea. They can kill insects that are their own size, but do better with smaller prey (and the prey dies more quickly).

An adult assassin bug lives for up to two years and females produce a huge number of eggs. They are about 2 millimeters long are purple, with white caps. To hatch, they need to be kept moist. You can either leave them with the adults or you check the substrate at least once a month for eggs and raise the nymphs seperately. Right after hatching, the nymphs are bright red with yellow legs, older nymphs are purple-black with black legs.

Even though they have a painful bite and can spray you with defensive liquid, assassin bugs are peaceful and not at all aggressive. I don't recommend handling them, but you can take them out of the tank with their hiding places or whatever they are sitting on without problems if you want to clean the tank.