Or HaChaim on Genesis 4:1/אור החיים על בראשית ד׳:א׳

1 ותלד את קין. She gave birth to Cain. According to our sages Cain represented the spirit of impurity and evil whereas Abel represented good. Abel's soul is supposed to have been reincarnated in the body of Moses (compare Zohar 1,54 on this passage). Abel suffered multiple injuries at the hands of Cain because Cain did not know where to inflict a lethal injury. The pains suffered by Abel in the process removed any vestige of evil that may have been in him so that when Moses was born he was totally good. This is what Eve hinted at when she said: קניתי איש את השם. She meant that the perfection of a man such as Moses who could commune freely with G'd, was due only to the sufferings undergone by his ancestor Abel.