It's because the people, who are against God in the OT, are always wicked persons and the phrase "they were very sinful" is not able to convince us about this. See, pedophilia, zoophilia, infant sacrifice, massive orgies and stupidity are not just "great sins", but also immoral acts.
Therefore God's energies seem to them as angry and evil and this is what the accountants and the Prophets have also spoken of, since they were speaking to such people.
Most people can't see that God is actually not "tempered" easily (for the matter, He cannot essentially become any angry at all). His face seems so mean because He was dealing with evil and pervert people. He was still the same loving Father for the ones who were following His ways.