Inflation means that a dollar that you have will buy less in the future. (Savings are worth less, and debt becomes less of a burden.)
Deflation is the opposite. A dollar bill in your hand will be worth more in the future.
One has less incentive to spend, and less incentive to borrow, during deflationary periods. This decreases the rate at which money is spent, which decreases the apparent money supply, decreases tax revenues, etc. Basically, it causes a recession and/or depression.
Since the government bonds sold to China are a debt that the government owes (and since our government essentially runs on debt in general), it would be harder for the government to pay those debts if deflation were to occur.
Therefore, the government has incentive to prevent deflation, which is what quantitative easing does.
There are also incentives to prevent runaway inflation, mainly because runaway inflation would wipe out savings and debts, essentially making everyone a pauper. (It doesn't matter if you're a billionaire, if your billions are essentially a stockpile of toilet paper.)