I think that Blake views God as two sided, good and evil. In the poem, The Lamb, he uses the lamb as a metaphor for Jesus Christ. The lamb is a gentle creature, which shows the good side of God. In the poem, The Tyger, he uses the tiger to symbolize Satan. The tiger is an aggressive and intimidating animal, which characterizes Satan and the evil side of God.
I think that William Blake's view of the nature of man is similar to his view of God. God can create kind and gentle people, who are symbolized by the lamb, but He can also make mean and aggressive people, who are symbolized by the tiger.