How did 甚 + 至 compound to mean "even"?
甚 = extent

至 = to

甚至 = to the extent of (literal meaning)

(甚至)殺人(也)做得出來 = (to the extent that) he will do killing (too) = (even) killing (too) he would do


The extended meaning of &quot;even&quot; in 甚至 is implied from the implied 也(too) in the [甚至 ... 也] structure
Note: the literal meaning of 甚至 (to the extent of) is as commonly used as &quot;even&quot;. e.g. 甚至不吃不喝 (to the extent of not eating or drinking)