Ok I'll answer this question based on the Artistotelian proof of God written by Edward Feser in his book Five Proofs Of The Existence Of God. I have to establish a bit of groundwork before answering your question so bear with me.
According to the proof, change is the actualization of potential by an object that is already actual (the principle of causality). If I have a cup of hot coffee, it has the potential to become cold, but it does not become cold unless this potential is actualized by the cold air in the room. The coldness of the air is actual, but it did not become actual until its potential to become cold was actualized by the air conditioner. Through this we can see that there is a linear series of changes that is dependent on time. One change leads to another which leads to another. However there is another type of series of changes that is not dependent on linear time, but rather can be observed in only a single moment of time: this is called a hierarchical series of changes, which I will now explain.
Take another example where a cup of coffee is being held by a table in a room. At any given moment of time, the table is actualizing the coffee's ability to be held up above the floor, but the floor is, at the exact same moment, actualizing the table's ability to hold up the cup of coffee. The table gets its ability to hold the coffee cup from the floor, meaning that if the floor is removed, the table can no longer hold the coffee cup. By that same token, the floor is held up by the foundation of the building which is held by the structure of the earth, but the earth is not held by anything. We can therefore say that the earth is the first cause in the series of changes which has an inherent ability to hold without having to be held up itself. It is therefore the earth that is actualizing the potential of the coffee to be held, and it is doing so through the multiple mediums mentioned. The earth is at the top of the hierarchy in the hierarchical series of changes.
This is how God operates in relation to the existance of things. A cup of coffee only exists insofar as the water which constitutes most of the coffee also exists, but why do the atoms/subatomic particles of the water exist in the form of water? After all, the hydrogen and oxygen atoms have the potential to form themselves in other ways, but at this moment it is formed specifically as water. Why? As I said earlier, potential must be actualized by something that is actual. In this case, the thing that is actualizing the water's potential to exist at a given moment is a purely actual actualizer that does not need to be actualized, or in other words, something that has the innate ability to cause existance but whose existance doesn't need to be caused. This is comparable to the earth which did not need to be held up yet possessed an innate ability to hold other things. The only difference in this case is that we are referring to existance rather than the ability to hold. The purely actual actualizer is at the top of this hierarchical series of changes and so it does not need to be actualized. This entity can be conceptualized as God.
Now, back to your question; you asked why God is responsible for everything good but is not to be blamed for anything bad. Well in the proof that I just presented, God has no potential component to him and so he is incapable of being bad. I'll explain. Something is said to be good when it is able to fully fulfill a potential inherent to its being. For example, a painter is good when he is able to fulfill his potential of mastering the art of painting. A painter is bad when that potential is, for some reason, not met. However, in the case of a purely actual actualizer, there is no level of potentiality. God's pure actuality means that he is not capable of missing the mark or failing to fulfill his "potential," since there is no potential that needs to be fulfilled to begin with. It can be said for this reason that God is fully good. As for any bad thing that happens, this is always the result of potentials that are not met, which, as we've established, has nothing to do with God's nature.
Wow that took a while to explain, but it's quite an interesting concept worth analyzing.