Evocation is a very good investment for a berserker. You get your ranged attacks, summons, utility spells, all in one skill. Of course, you have to actually find the items to evoke, but that's not hard.
For example, the lamp of fire/fan of gales/water elemental thingy only get better with more evocation skill, and they recharge over time(actually over XP gained, you can't just wait it out)
Having your wands of fire/ice etc do more damage is always nice.
If you're lucky, you may find some rods too(as a Trog follower, if you get an acquirement scroll and ask for a staff, you'll get a rod). They're basically rechargable wands, although they have different effects.
As far as using the exec axe goes, it depends on a few factors.
First off, what is the executions axe's brand, and enchantment? If it's a -5-5 executioners axe of venom, or something equally crappy, just forget about it. If it's at all non-crap, you might want to use it.
Do you already have a decent axe? If so, you may want to stick with it for a while, until your axe skill is higher.
What is your axe skill right now? You may want to focus on it more, to get the exec axe up and running faster. Making things dead faster is a good form of defense.
I'd say, if you don't already have a broad axe or battleaxe, just switch to the exec axe right now, and focus all your skill gains on axes. Once it's taking 1.2 turns or less per swing, you can go back to training your dodging, fighting, armor, etc, while still training some axes. You're going to need 26 axe skill for min delay, but don't feel that you have to rush to that goal. You definitely are going to want some fighting, dodging, and armor skill, at the very least.