When love beckons to you, follow him,Though his ways are hard and steep. And when his wings enfold you yield to him, Though the sword hidden among his pinions may wound thee. .... My fav poet's words....love is like the wind between the trees. Love is wonderful and love can be terrible, it is an emotion.

I think you have mentioned this beautiful poem before, but I have forgotten what it is called?

Aristotle would put it above the mean of like but bellow the extreme of destructive obsession.
Hm. That is not a bad way of saying it.

What about love for other than persons? Pets? Countries? Books?

It's just that, love for things other than another person can be just as intense.
I think so too..

Of course not, one can love many different people and things all at the same time. I love my relatives, my husband/owner, my friends, my cat, etc. They don't have to take turns. lol

Well said ;-)

Or to kill for love in other situations?

Avenging the death or rape of one's loved one when catching the individual responsible in the heat of the moment? Yes.

Is that love or revenge? Is revenge out of love? (I mean, in such a case as this).

I am sorry I am still too vague with some of the other questions, maybe I can come up with something more specific.