Allow me to clarify. While you made clear the flavour points behind this card, you still haven't addressed the mechanical inelegance of the card. When I say that the abilities have nothing to do with one another, I mean that they don't work together in a way that enhances the game. Take a look at
Magnetic Magician. It's a card I created recently. Note how the two abilities play off one another. That sort of interaction should be present on most cards with multiple non-keyword abilities.
As far as the flavour goes, take note that only somebody who was previously aware of the story snippet you've written would understand why these abilities are present on the same creature. Furthermore, the principle of repentance is not characteristic of either black or green; it is characteristic of white. (See Repentant Vampire for one example of how this concept can be conveyed.) An excellent way you can generate flavourful abilities for a card is to select an attitude you want the card to convey. The two attitudes you've expressed that this card should convey are remorse and tranquility. Those are excellent qualities and could make for a very interesting card.
Let's take a look at your card's abilities and determine whether or not they actually convey the ideas of tranquility and remorse. I will also comment on whether the ability is of a reasonable power level.
Creature spells may not be played.
This is vaguely indicative of solitude, which may result in tranquility. However, the power level of this ability is very high. The nearest comparable card I can find is Feroz's Ban:
Feroz's Ban
Artifact
Creature spells cost
more to play.
Note that it is an
artifact with a converted mana cost almost as high as that of your creature. Furthermore, its ability is much weaker than yours and is the only ability the card has. This sort of comparison shows that the ability of "Creature spells can't be played." is much too powerful in this instance. You would be better off finding another way of conveying this creature's desire for tranquility. One way might be to state "~This~ can't be the target of spells or abilities." That appears on many creatures, some of them quite huge, such as Plated Slagwurm. Furthermore, it is a reliable green mechanic. Naturally, if you wish to include other beneficial abilities on this, then you may not be able to make it quite as huge as Plated Slagwurm.
Zaolac, Tranquil Evil cannot attack unless you pay 3 life.
As previously noted, I like this ability. I think it conveys the flavour of repentance very well.
GGGBBB: Zaolac, Tranquil Evil gets +3/+3 and gains shadow until the end of the turn.
This ability comes out of nowhere. I can almost see the idea of paying some cost to give the creature Shadow. That might be a little bit appropriate for conveying the Zaolac's desire to be alone. However, you've already got another ability that conveys this. Furthermore, the +3/+3 comes out of nowhere. Your argument that it comes from Zaolac's desire to protect the silence is inconsistent with the repentant quality you're trying to build up.
The last comment is on the numbers. Your casting cost is huge, your power and toughness are huge, and the cost you've given the ability is huge. Tone this down a little bit and you might have something reasonable. If I were going to take this flavour and run with it, this is the card I'd create:
Zaolac, Remorseful Horror
{G}
Creature -- Horror Legend
Zaolac, Remoreseful Horror can't be the target of spells or abilities.
Zaolac, Remorseeful Horror can't attack unless you pay 3 life.
(This cost is paid as attackers are declared.)4/4
