Is it possible that if you have the skill sneak at 100 that you can steel armor

For example That Deadric Armor of Diviath Fyr bij Tel Fyr

Or is there another way to get that Deadric Armor but not by killing him ???

Yes. Use a combination of a disentegrate armour spell in conjunction with a Calm Humanoid spell to keep him in check. After about 2,448 uses of both spells, all Fyr's armour will be broken and displaced and you can attempt to pickpocket him.

OR, you can just get the other pieces of armour in the game that do not require potentially annoying a Main Quest NPC. There is a far higher chance of you attaining those than breaking Daedric armour, which has a very high HP count.

Use the UESP to locate the other pieces.

If you enchant a weapon in the editor with a high Calm spell and a high Disentigration spell, it will greatly ease your endeavors.