I love this trade for the Hawks, but can also appreciate why the Chiefs made this deal as well. Lets look at both players, from the viewpoint of each team.
Kevin Pierre-Louis (KPL)
It's no secret that KPL was over-drafted by the Hawks based on his freakish SPARQ scores before the draft. Based on his tape, most considered him a late round or UDFA prospect prior to the combine/pro day period. But he recorded the 2nd highest SPARQ score of any draft-eligible player in 2014, just behind Ryan Shazier, and just ahead of Khalil Mack. Unfortunately for KPL, at his best position in the Hawks D (WLB) he was stuck behind KJ Wright, and his struggles with reads and pass drops in coverage didn't allow him to take advantage of what was a wide open SLB position since Bruce Irvin's departure. He was unable to compensate for his middling LB play with stellar play on special teams, instead producing serviceable-but-unspectacular ST's play. In that regard, there was little-to-no upside left in KPL for the Hawks, and as this will be the last year of his contract before becoming a UFA after the season, he was unlikely to suddenly play and produce at a level that would warrant a contract in free agency large enough to even register in the comp pick formula. The Chiefs clearly saw an opportunity to try out KPL as an ILB in their biltz-heavy 3-4 scheme, a role more suited to KPL's abilities where he'll have fewer reads and pass coverage responsibilities, and be able to shoot gaps as a pass-rusher more often - something he did well at Boston College. Ultimately this will serve as a one year 'try before you buy' opportunity for the Chiefs, who'll be able to gauge whether KPL could be a potential replacement for Derrick Johnson at the LILB position before possibly trying to lock him up on a new contract before he hits free agency.
D.J. Alexander (DJA)
The Chiefs' scout who ID'd Alexander said after the 2015 draft that their selection of DJA was ostensibly made entirely based on his special teams potential. In that regard, they nailed the pick. But as a LB, DJA was completely miscast as a 3-4 ILB in KC trying to fill gaps against uncovered OG's in the running game, and thus offered no plausible long-term potential as an option to eventually replace Derrick Johnson (who could retire after 2017, and who has zero shot at seeing the $8M on the final year of his contract in 2018). The Chiefs will always have stellar ST's under Dave Toub, and could afford to sacrifice and parlay a player like DJA into a potentially more serviceable LB prospect better-suited to their defensive scheme in KPL (who is also an experienced and athletic ST's player as well). For the Hawks, DJA will slot in behind KJ Wright as the backup WLB in their 4-3, a role he's much more naturally suited to. The backup to an All-Pro-caliber OLB has to really bring it on ST's, and DJA can and will certainly do that as an elite 'core four' ST's player (Pro Bowl last season)....certainly at a level much, much higher than KPL did. DJA was said to be a very popular personality in the Chiefs' locker room, while KPL has unfortunately had to manage his documented challenges with depression. DJA is also under contract for two more seasons (vs. one for KPL), ostensibly at the minimum.
So again, each team is getting a LB who is better suited to their particular scheme than the player they're trading away. The Chiefs were looking for a player with a higher ceiling on defense due to the impending closure of Derrick Johnson's career, and the Hawks were looking for an upgrade in ST's from their backup WLB who, regardless of his name, will be blocked by KJ Wright. In that regard, both teams succeeded and did well here, the Seahawks in particular.