The -90% sunder charm for lightning is effecting player resistance instead. My lightning resist shows up negative now since equipping the sunder charm. Shouldn’t this just be for enemy resistance?
Buddy , you absolutly dont know how sunders working.
First, all sunders break enemy resistance, so you doing 5% damage against sundered enemies where previsiously you do nothing to them.
Second youre wearing the worst sunder because that you can break the immunes the(-70)-(-90) resist penalty is youre character penalty 
The best sunder is the -70 sunder, and this is youre penalty wearing the sunder charm, not the enemys resistance reduction
This is how sunders working after Blizz put them in the game, this is not a bug, you dont know how Sunders working.
Plus you deserved that you sucks with sunders, i hate lazy people like you cuz you can google or see on Youtube the content creators as MRLlamasc, Ginger Gamming Mentor, Iceman etc. etc.
The game is 4 years old, search and read the topics noob, before you whine in the bugreport topic.
Yeah man, that’s the work of sunder charms, you take a penalty for sundering an enemy’s resistance which is why you see the negative. I google’d it instead, but im not about to sit here and demean you for reporting it as a bug, as any dumbaxx player thats been playing 20+ something years would know that other players would go to the source to ask or report rather than to search. SOME Old players are an embarrassment to the game and it is most unfortunate, not enough sunlight i guess. Stay kool little bro, I hope u got ur answer
that’s exactly the idea behind sunder charms. They provide a great power at a great cost.
Ditto what has been said. Prior to Sunder Charms, Monsters were immune to certain attacks. These immunities were associated to their resistance. Anything 100 or above = immune to that respective element and are quite common on Hell Difficulty. Monster Resistance can be found on the outdated Arreat Summit if you’re curious or want to see what you’ll be up against on any given Act.
Anyways the only way to break Elemental(Fire/Cold/Lightning) immunities was to use Conviction Aura(via runeword Infinity, which is really, really expensive or from the Paladin themselves(hence playing in a party has its merits!!)) or Lower Resist curse(which can be found on Necromancer wands with charges for the Curse or by playing with a Necromancer). Any time you are attempting to break an immunity, skill works at 1/5th effectiveness. So the level 12 Conviction on Infinity that normally yields -85 to Enemy Resistance only yields -17. That meant that any Monster whose Resistance was 115 or lower could be broken by it. Anything 120 or above could not and many Monsters in the game can be permanently immune no matter how much -Enemy Resistance you had and was most prevalent for Cold for obvious reasons.
Fast-forward to the introduction of the Sunder Charm. Sunder Charms when equipped in your backpack/inventory allow you to effectively ignore Monster Immunity for that respective Element by setting their Resistance to 95. Any additional sources of -Enemy Resistance on gear would drop it further at 100% of face value(hence items like Griffons Eye or runewords like Crescent Moon). Additionally, Conviction, Lower Resist and Cold Mastery(Sorceress) would apply their effects at 1/5th the listed value to Sundered Enemies. Ex: Infinity would lower it a further 17. Enemies that are not immune feel the full force of everything you have.
At the cost of Sundering Enemy Resistance, your resistance to that Element is penalized by 70 to 90. Good Sunder Charms have the minimum penalty of 70 while bad ones have the max of 90. If that’s all you got, you have to live with it but I advise you replace that Charm with one that has a lower penalty at the first chance you get.