Wow, a lot of deep questions.

What is love? Love is a feeling that you know when you have it. I can't explain any better than that.

How does it show itself? This depends on the individual.
How does it change a person? It doesn't, you're still the same person as before one was in love, although you may do stupid things for a while.
How does it change your perception and your ideas of another person? In the early stage, infatuation, you might overlook all their faults. Later one, while still being in love, you know their faults and love them anyway.

What about love for other than persons? Pets? Countries? Books?
Eros is one kind of love, romantic, there is also brotherly love, parental love, love of things (I'd say this is more of a strong like, though, where we say love but it's not with the same attachment people feel to other people). Pets are in between things and humans I'd say. I love my pets more than my favorite books, but less than my human family.

Is all love monogamous?
No

Can you love only one? (at a time, or in your life) No, I love lots of people, just not in the romantic sense.

If you fall in love with someone, must you fall out of love with others?
No, not necessarily. I have only been in love once, but I think it is possible for others to be in love with more that one person at the same time (eros).

Does love and jealousy follow each other, or are they seperate

I think they are at times related, but you can be jealous with out love and you can love without jealousy.

Can you love without jealousy? Yes, some people can.
Can you be jealous without love? Yes, definitely.

Is crime passionel a sign of love, or of love dead? It's a sign of being unable to control your actions.

What causes crime passionel? Jealousy and lack of control, if that means what I think : Crime of passion?
Does society support the idea, and, if so, why?
Parts of society do, but not all of society. I believe this is changing to be less forgiving of people who commit those crimes.
Or did it, and is now changing attitude? If so, is that ok?
Yes. I keep answering the question ahead of time.

Do you think it is understandable/ok to kill the loved one? Not at all. Just leave if you can't stand it. There is no right to kill them. That is just evil.

Or to kill for love in other situations?
Only if your loved one is directly threatened and you kill someone to protect them from the same fate. I also believe in mercy killing (euthanasia) in cases of terminal illness, and believe that to be an act of love.

D/s and love

Can you have a D/s relationship if there is also love? Yes, I do
Can you have a D/s relationship without love? Other people do, so I'm sure it's possible.