OK, I'm perhaps missing something here, but I can't see how the method of killing and the need for mass killing (human and non-human) can be interpreted as a sign of love for the very beings god claims to love.
I really need someone to spell it out to me here.
We have to put this back into the correct context. When talking about a literalist view, at least the one I grew up in, one sees God as being the creator. The thing we heard over and over again was that God was an artist with clay, and he formed us. God then has every right to do as he pleases.
So God created the Heavens and Earth, and he makes it perfect. However, because of Satan (note, Genesis does not label the serpent as Satan, but that is how we were taught), who deceived Adam and Eve, evil came into the world. No longer was this world perfect, but it was tainted. Because of that, man suffered.
God tried to make things right, but it just didn't work. Things were getting out of control, and the world was going to hell in a hand basket. So instead of allowing things to get even worse for all of us, he decides to just end it. He no longer wants to see his creation, that he loves so dearly, be used as a tool of Satan, and basically destroyed from the inside out. He doesn't want to see the evil that his creation is going down.
So God decides to just wipe the slate clean. Instead of allowing his creation to be used for evil purposes, he just gets rid of it. It's not something he really wants to do, but he thinks it is for the best. However, he sees Noah, who is a righteous man, and who really isn't being sunk into this evil world.
So he gives Noah a mission. Build an ark. Save enough of the creatures so that life can continue. And warn your fellow man what is going to occur. So he spends a great deal doing just this. He warns man what is going to happen, and he continues to build the ark. But everyone just mocks him. Their hearts are stone, and they have been turned over to evil.
Finally, Noah completes this ark, he puts all of the life onto the ark, so everything can restart. And then one final time, he offers to save everyone. But they reject it. And the rain comes. Basically, it is God trying to once again create the perfect situation for the creation he loves so much. In order to do that though, he has to wipe the slate clean. He is simply doing what he thinks is best.
After it is all done, God guides the recreation. At the same time, he sees the destruction that was caused, and promises not to do that again. That instead he will find other ways in order to make things better here on Earth.
Looking at it from this point of view, God was just doing what he thought was best. His creation had been turned over to evil, and he didn't want to see what he worked so hard to create, be used in that manner. So he starts over. It's a struggle for him, but he does what he thinks is best. To save humanity, he first has to destroy it.
I understand that the story is meant to ultimately convey god's love, but as before, it's the methods employed, and on such a scale, that I can't get past. If I were to humanely kill a fatally-injured animal, I wouldn't throw it in a lake and let it drown - I would euthanise it quickly and painlessly.
Edit: you said that I am focussed on the evil in the story. That's correct, because we are discussing how a loving god can use such tactics to demonstrate his love.
The problem is you're thinking in modern terms. Ancient people thought differently. You drown the entire planet, and it is instantly done. There is no mention of suffering. It just a restart.