Demorrio Williams has a pretty interesting story. According to Wikipedia, Williams signed with a rural community college right after high school but didn't stay long. Born in far east Texas, Williams worked in an oil field after leaving the community college early. After walking on to Kilgore College's team, the head coach moved Williams to linebacker from cornerback, sparking William's future football career.
Williams leveraged his time at the small Kilgore College into a two year gig with the Nebraska Cornhuskers. Williams played in fourteen games in 2001, starting eleven of them. In Williams' senior season in 2003, he started all thirteen games for the Huskers, amassing 83 tackles.
Demorrio played almost entirely on special teams during the 2004 season for Atlanta, after being drafted by the Falcons in the fourth round. He started sixteen games in 2005, racking up 127 total tackles. Williams lost time in 2006, as the Falcons had a breadth of talent at the linebacker position. He only started ten games in '06 but still managed 90 tackles. Williams injured his pectoral muscle after the 2006 season and missed five months during the off-season. He started every game in 2007 but shared time with others throughout the year.
Williams' strength is playing the weak side, which means that if he did become a starter for the Chiefs that Nap Harris would be benched. Williams' greatest asset is his speed, which was measured in the 4.5/40 level at the NFL Combine in 2004. He was the fastest linebacker coming out of of the '04 draft but he's often criticized for his small size (6'0", 225 lbs.).
After the 2007, Demorrio Williams was sort of the odd man out in the Falcons' linebacking corp. Coaching changes amid basically a franchise upheaval in the wake of the Michael Vick incident meant that Williams wasn't long for a team with a solid group of linebackers, even without Williams in the mix. He was yanked during the first game of the '07 season after showing some rust from his pectoral injury. Still, there wasn't a lack of suitors for Williams in 2008. The Bucs, Chiefs and Bears all had visits with him.
I think it's safe to assume that Williams will compete for a starting spot. One of the most consistent knocks on him I found throughout my research was his tendency to get thrown around because of his smaller size and his tendency to over pursue. When you're as fast as Williams is, it has to be tough not to over pursue.
Besides the Chiefs' current starters at linebacker -- Edwards, Harris and Johnson -- the cupboard was filled with very unproven linebackers who have been playing special teams. Pat Thomas, Johnny Baldwin and Mickey Pimental and don't have a start at linebacker between them. With Demorrio Williams, the Chiefs added a proven linebacker with a solid number of starts under his belt that they didn't have to break the bank on. If he does end up starting, he'll fit well between the still-young Derrick Johnson and the veteran Donnie Edwards. The linebacking corp can always use more speed.