Picasso has being highly analyzed through his hand writing, here is a quite accurate description of certain issues regarding his terrible temper.

The first thing we notice in Picasso's handwriting is the emotional volatility. Picasso was emotional dynamite and there is evidence of a somewhat manic quality. The sample below reflects clearly how he felt at the time of writing -irritable, angry and dictatorial. The sharp choppiness, the angles and distortions all add thrust to the total picture.

In his handwriting we can see the almost chaotic variations of slant, wild disorder and a complete disregard of the conventions of handwriting. Contorted words surrender most of their legibility to the authority of his dynamic will. Great waves of unruly passion stir in the depths of the darker strokes. His mood is volatile, angry and brooding. Note how the word "mort" or death is larger than all the other words. It shows frustration and anger amidst the chaos and creativity he rejected every vestige of conventionality. Picasso was a law unto himself. His handwriting shows it.

His daughter Paloma Picasso was interviewed in a TV program she was not kind towards her father. She said he was a dictator, abusive and oppressive towards women. He did not care on how women should be treated and was extremely cruel and intolerant. It is believed he had a mental disorder.

His total self-centeredness. He made everything - and that includes his personal relations - a subject for his art. He was obviously hardly capable of empathy on a personal level.

He was born willful and stayed that way + the other answers:)