It's not true that Christians don't take the old testament seriously. Sure, you might hear somebody unlearned saying that, but Christians view the old with the enlightenment of the new. That is to say, understanding the old in light of the new, namely Christ. Given that the old testament is comprised of many different books with many different genres, it's impossible to talk of it in the same way one would talk of the Qu'ran. For one thing, the Bible is full of context, including historical names, places and dates, and large parts of it record the actions of men, one tribe upon another, good and bad, both faithfully written down. The Qur'an isn't contextual, and to understand that the Medina surahs come later in historical chronology than the Mecca surahs, you have to do further research than simply reading the book. The Qur'an's murderous streak comes directly from its author (although not written down by Muhammad himself, for he was illiterate), and the militaristic commands therein to fight unbelievers abrogate earlier verses which emphasise peace and a lack of compulsion in religion (not surprisingly, these verses come from a period in history where Muhammad and his followers were the minority in Mecca).
In the Bible, we have the clear emphasis of God's Holiness and righteousness throughout the old testament, demonstrating that it was only by God's grace that he dwelt with a sinful people (the ancient Israelites, so named after Jacob) at all, and this finds its conclusion in the Son of God laying down his life to redeem any who would believe in him from the wrath of God's righteous judgement on sinful creatures. The clear trajectory of all the scriptures of the Bible is towards grace and mercy, which is only recognised as grace and mercy by the prior revealing of God's righteousness and judgement against sin.