If a model is in a melee with 3 other models (one friendly, two unfriendly), is only in base-to-base contact with one, and then defeats that model via a Close Combat action, can they then Slide in order to be in contact with the other unfriendly model?
Or, since they’re now free of contacts to unfriendly models, would they have to charge to re-engage the other unfriendly mode?
Or did I play this situation entirely wrong, and having 2 models per side of a melee doesn’t work this way at all?
A model can use Slide at the end of a Close Combat action if (a) they are engaged with an opponent, and (b) it does not break engagement with any opposing model. Slide is part of the Close Combat action so there is no chance to Slide if the attacking model is no longer engaged with any opposing models.
It’s a bit tricky to tell from the description as it depends on who is where but I think what you’re describing is that the attacking model hits and removes the only opposing model it was engaged with. If that’s true then the model could not use Slide after the opposing model is removed as there is nothing to Slide around. They would need to Charge or Move to engage with the remaining opposing model (which is currently engaged with just the other friendly model).
There is a situation when a model can Slide around a different model to the one it attacked. Using the diagram at the bottom of page 25 of Rules of Play, if the Behemoth attacked B and removed it, the Behemoth could then Slide around A as it is engaged and would not break contact with an opposing model.
(Note that a model can only be ‘engaged’ with opposing models and not models on its own side.)
Hope that helps.