Any way to make event horizon/speghettification work?

I'd not route into them.

But I've been thinking about annointing it eventually. The chaos res loss isn't a huge deal. I'm on mid-tier gear in HC at level 80 and I have all resistances capped, chaos included.

The only reasonable use I've seen of the node is through monk when using Hand of Chayula or Mantra of Destruction.

Otherwise it feels like to me you'd need a lot of leech that's only really capable through Blood Mage and her life remnants

I've mostly only seen spaghettification be relevant as an anoint or megalomaniac node for attribute stackers who might also put an against the darkness in that jewel socket which hits spaghetti.

Assuming you're playing infernalist

Bottom line up front:

Infernalist may not be the best for chaos damage, but you can make this model work in a build.

Here's my take on how to solve event horizon:

Stack mana, take MoM, EB, CI, and Dark Entries before event horizon and sphagetification. I recommend a mahuxotl's machination 600% with 2 max mana soul cores, a +3 spell skills/8%max mana amulet, and a +5 spell skills level wand.

Stun threshold and elemental ailments will be important.

OR

Original sin amethyst ring (converts all elemental damage to chaos damage, you don't need a good corrupt roll on this ring as excess of 100% does not do anything AFAIK) and cast only elemental spells + energy shield stacking

For the chaos infernalist witch, you are in a bit of a tough spot with event horizon since you lose hp/es each cast, but there's a few ways to mitigate this

1. You need Chaos Innoculation (CI)

You lose chaos resist from spaghettification. CI makes you immune to chaos damage, with the added benefit of removing a stat that you need to stack on gear (this makes things cheaper more or less!) 

2. With CI you need a ton of ES or to take mind over matter (MoM). Since you want event horizon, energy shield will be lost on chaos cast, which is where Eldritch Battery and a big mana pool come in to act as your health pool instead. 

3. You will struggle with cast speed and need mana regen nodes. This is where other class ascendancies start to make a bit more sense than witch.

4. You need either fantastic resistances and damage mitigation (80% max res plus) OR a massive mana pool.

Mana stacking items are super pricey, so I I recommend a mahuxotl's machination 600% with 2 max mana soul cores, a +3 spell skills/8%max mana amulet, and a +5 spell skills level wand.

This paired with decent gear (all 20 int and 100 mana or more, probably in the 20ex range each except a wand and amulet) should put you at around 5k mana by level 90, which is enough to feel beefy

5. Finally a positive!! You're right next to the Resonance keystone node. With this and snipers mark, you could easily do big ceits with power charges! That would synergize well with the witch crit ascendancy, but then you have to worry about ignite damage. Oh wait, your HP is 1 and you have no ES, so that's 0.15 damage ignite to yourself in exchange for 30% MORE crit damage bonus. Huge. Also, you're paying past some great crit nodes with this, so it would be easy to scale crit damage. 

6. If this exact same build was on a stormweaver, you coould have permanent arcane surge and that would increase your cast speed, mana regen, and spell damage for each 15 mana you have, freeing up a ton of passive skill points. Also, the all damage contributing to shock ascendancy node gives you a free 20% more damage increase for anything that stays alive!

Okay I've shared almost all my discoveries, hope these help! 

I have a Lvl 90 witch and lvl 90 sorc both originally chaos builds

I have spent lots of time (10+ hours of staring at the passive tree) trying to get my chaos witch to work. Ultimately, it ended in me with a chaos sorcerer mana stacker and a melee witch hp stacker that is absolutely wonderful and clearing all content with gear under 10d