One could come up with very different justifications behind the fact that Shirley Jackson places Mr. Steve Adams at the head of the crowd waiting to stone Tessie Hutchinson.
Old Man Warner was saying, "Come on, come on, everyone." Steve Adams was in the front of the crowd of villagers, with Mrs. Graves beside him.
I tend to believe that Jackson was bringing into her text a small bit of Biblical ideology. To justify this, examine his name: Adams. Adam was, according to Christian ideology, the first man. Given that Steve Adams bears the name of God's first human, who better than to be heading the crowd?
Outside of that, Steve Adams was the first one to draw during the lottery. Families drew according to last name (in alphabetical order). If he is the first to draw, it would seem that he would be the first in line to stone the "winner."
Another point to raise is that Mr. Graves is standing right next to Steve Adams. Symbolically this makes sense. Tessie will be sent to her grave at the hands of the villagers.
Therefore, a man leads Tessie to her grave. What better statement to make than Adam (the first man) leading the winner to her grave?
I feel that the Adams were two of the only people to question the meaning of the lottery. Both Mr. and Mrs. Adams discussed how other towns in the area had given up the lottery, and just to bring that point up during the lottery show's that they may have negative thoughts towards the act. I believe Shirley Jackson has Mr. Adams cast the first stone to show that though people may speak up when they think something is wrong in small groups, when it comes to peer pressure even those against acts of violence will join in to be part of a mob. (Historical Examples: The Holocaust and the Japanese rape of Nanking during WWII, just before Shirley Jackson wrote the short story in 1948).