Kaczynski had the knowledge for the complex level of encryption taht was used in the letters. However people who use bombs rarely have the courage for up close killing.
But Kaczynski never developed a bomb until 1978. He was angry enough to kill long before that. What precipitated his desire to kill was the shame he felt over seeking a sex-change operation, which in itself was the product of extreme sexual frustration. That was in 1966.