The cross is not a bad thing, it is always used positively in the Hebrew Bible. Ezekiel is an example, but the meaning of Cain is even deeper. When Cain killed his brother, and God punished him for it, he asked God for mercy, fearing to be killed himself. God then made a sign on Cain's forehead so that he would be saved from it. What kind of sign was this? Here it is not the word Tav, but the word Oth, this word consists of the three letters Alef, Vav and Tav. These three letters(words) are names of the alphabet, and accordingly have their own meanings. Alef(ox head) is the first letter and symbolizes the highest rank and stands for strength, it means God. The Hebrew words like Eloah and Elohim begin with the letter Alef. Vav represents a nail. And Tav, as I said, is a cross. If we combine all three it means "God is nailed to the cross". So what God made on the forehead of Cain was the image of a cross on which Jesus hung. A powerful image. Again, the cross is salvation.