The battleship of the Vasari capital fleet, the Kortul is first and foremost a strong offensive capital capable of dealing major damage to enemy formations without using it's abilities at all. This alone makes it a strong choice, although the strength of some of the Vasari's other capitals does not make it as good of a choice for a first capital as the TEC's Kol. Still, if you want a ship that is primarily combat-oriented, this is the one to build, and it is in fact the most offensively-oriented capital out of all the battleships in the game.
The Devestator has three offensive abilities, all of which are good. The first two are Power Surge and Disruptive Strikes. Power Surge dramatically increase the firing rate of the Devestator's weapons and also increases shield regeneration by a small amount. When fully upgraded, Power Surge has a long duration and increases the rate at which the ship's weapons fire by four times. This is an insane offensive combat buff, and it's good when the Devestator is used as a straight-up combat ship. Disruptive Strikes is a passive offensive ability that gives all of the Devestator's weapons a chance to destroy enemy anti-matter and increase enemy ability cool-downs. Stacked with Power Surge, this makes the Devestator a wrecking ball against support cruisers and capitals.
The third offensive ability is Volatile Nanites, a Ultimate Ability which increases damage done to enemies in its area of effect by 30%. It also causes enemy ships which explode to deal out 150 points of damage to nearby enemy vessels. This is an outstanding combat buff which can dramatically increase overall fleet damage against a clump of enemy vessels. It is particularly good against frigate blobs, as each enemy frigate that pops does damage to nearby frigates, causing a chain reaction of explosions.
Lastly, the Devestator has Jam Weapons, which disables the weapons of enemy strikecraft for a long duration. This is not useful if you don't actually have a strikecraft counter, since it does not actual damage to the strikecraft. When used against large formations of enemy strikecraft which are fighting your own strikecraft however, it will turn the tide of battle.