Why shouldn't Hecarim trust Mordekaiser?

On the League of Legends Wikia I found out that an Altar on the new Twisted Treeline says to Hecarim "Mordekaiser is not to be trusted, Hecarim!"

Why? I can't find anything in their lore about their past relationship.

In an AMA about the new Shadow Isles patch posted on Reign of Gaming's web site today, that same exact question was asked:

Q: Why is the left altar saying to hecarim when he caps it, that he should not trust Mordekaiser? [...]

A: [...] We have developed an epic and comprehensive story for the shadow isles that we hope to fully explore in the future. The interactions at the altar are giving hints at this storyline. I am glad you are curious as to why the spirit is telling Hecarim he should not trust Mord, I am not going to reveal that today but it will become more clear over time.

So that's all the information available at the moment. We'll see in the future!

Well, it is clear that Hecarim was a type of soldier or cavalry. What I would like to know is if he is actually a centaur or not? There is so much to Hecarim and Mordekaiser and their relationship is the only interest in which I share to the lore of league of legends. Most interesting part of the Shadow Altars is it quotes "Hecarim, never forget who you once was" which clearly means he has had some type of relation to the shadow isles perhaps before it met cataclysm. It cannot be the theory that he was conjured by a hateful necromancer if he has a past life and people cannot assume he is ancient if they cannot determine his age.

The other interest in too the Shadow Isles is Mordekaiser. Some people are coming up with the theory that perhaps Mordekaiser was once a "murderous emperor/king" of the Shadow isles. There is little information to help support this rather interesting theory other than the spectral beings from the shadow altars which quote "I serve you Mordekaiser because I must" also "I gladly serve you Mordekaiser".

Now, the blade of the ruined king has been theorized as Mordekaiser's past weapon due to the fact it has life steal which is the same as Mordekaiser's Ultimate ability. I am not sure but in the shadow isles Mordekaiser must be both powerful and influence and supposedly not to be trust which means he must be planing something rather sinister

Not sure if this helps but in German "morde-kaiser" breaks down to "murder-emperor" or "murdered-emperor" so depending on the tense his very name could explain if he was a Murderous King or a Ruined King.