In every legion there are individuals who should have risen to greatness if they had lived in any other era. In the Dark Angels, Luther would have undoubtably been placed in charge of The Order on Caliban in the absence of Lion El'Jonson. Kor Phaeron and Erebus would have risen high on Colchis in the absence of Lorgar Aurelian. Sigismund is amongst their number in terms of deadliness if nothing else. His legend is writ large across the Horus Heresy and before it. For without doubt, there are few who could match his skill in combat. Sure, Kharn had the rage, Lucius had the skill, Abaddon had both combined, and Dynat the mastery. But of all the Angels of Death, Sigismund is truly the most able to claim that title short of the Primarchs. As Extermination puts it: "Beneath the Primarchs, there has perhaps never been a more skilled warrior in combat", and Sanguinius to quip: "…less my brother Dorn's champion, and more Death's himself...". Lucky for the Imperial Fists he was a loyalist really, and a crusader to boot. Imagine if he were a Night Lord instead?!
His rules really back up this Angel of Death assessment of his background. From the outset of close combat, he can re-roll charge distances to ensure he and his unit get in to melee, as well as sweeping advance rolls, and gain +1I in the first round. An additional point (that is almost by-the-way flavour) is the ability to have Templar Brethren as troops.
But when he does get in combat, he starts to get highly deadly. Not only must he accept and receive challenges, but he gains the instant death rule that force invulnerable saves to be re-rolled (if applicable). And opponents will be doing that as his preferred weapon is AP2 and S+2.
His final special rule is the Slayer of Kings warlord trait. This rule means that the army can gain a bonus to combat resolution for the entire game if Sigismund slays the enemy warlord (plus an additional victory point, if required). This really means that Sigismund needs to be played in such as way as to directly challenge the enemy warlord whenever and wherever possible.
As above, the targeting of the enemy warlord is the way to go for Sigismund. But how will he get to where he needs to be? Is it with a body guard on board a land raider, rhino or other conveyance? Or perhaps something else thanks to being a Master of the Legion?
His artificer armour and Iron Halo ensure that he will at least somewhat survive against opponents, and unusually for named characters in 30k, he also has Eternal Warrior (coupled with fearless, and Adamantium will). His other equipment and rules are what you might imagine, apart from a master crafted bolt pistol. And did we mention 4 wounds? Sure, he costs like a land raider, but in my opinion, he is worth it.
Look at it this way, there's only a few enemy characters he need fear. Eidolon comes to mind. And he can even go toe to toe with some of the weaker primarchs (certainly the T=6 ones with a poor invulnerable save) and actually stand a chance. I can't say that about too many other characters.
Hence, overall, use him as a natural-born killer, but ensure he's got a land raider (or similar) to ride around it, and a half decent bodyguard to take some wounds along the way. His ability to take Templar Brethren as troops is good, but not the real reason to take this Angel of Death.