Eve also knew as well as Adam that they were not to eat the fruit. However, not only did she disobey God and eat the fruit, like Adam, but she actually convinced Adam to do it too (believing a snake's word over the word of God). That's two transgressions versus one. Whether or not you want to argue that the disobedience should have been punishable as well is something in its own right, but your argument that Eve was less guilty than Adam is wrong. Eve committed an additional transgression, Adam only committed one.