can anyone correct this sentence in case it´s wrong? thanx)

history teaches us that humans can easily and suddenly start killing those around them, even friends and relatives, and still keep a calm conscience. all they need to do so is seeing others do the same.

history teaches us that humans can easily and suddenly start killing those around them, even friends and relatives, and still keep a calm conscience. all they need to do so is to see others do the same.

thank you, vicente.
can i also say "all they need to do so is see others do the same"?

Yes, it can be said that way but I wouldn't write it that way. In speech no one notices. In writing it is permanent.

When you say "what do you need to do", the answer is "you need to see". If you say "what do you do", the answer is "you see".

thank you again, vicente��