Learn English – Idiom for someone who got a tiny fraction of power and abuses it

Someone who kisses up and kicks down, is used to refer to the context you describe:

• I heard the phrase “kissing up and kicking down” first during discussions about John Bolton, a Bush nomination for the US representative to the UN. Its meaning is probably obvious, but in case it isn’t, here’s a very brief explanation. It means being super nice to people with power over one, and taking it out on people over whom one has some power. The power needn’t be much more than a matter of rank or class. Someone who is rude to the cleaning staff, while super nice to higher administrators is following the pattern.

Note this neologism, Bootlickocracy:

One persistent and defining feature of such an environment is dominant use of "kiss up, kick down" principle by middle level managers. We will call such an environment a bootlickocracy.

The prevalence of bootlickocracy in large IT organizations can be viewed as a side effect of dominance of aggressive but incompetent managers who are colloquially called "empty suits". More often they are authoritarians and very often Double High Authoritarians).

Source:http://www.softpanorama.org/Skeptics/IT_skeptic/bootlickocracy.shtml

Source: http://feministphilosophers.wordpress.com/2013/04/04/kissing-up-kicking-down-and-tenure/

You can consider welch or welcher. These terms are used for people who fail to repay a small debt. The debt is usually a betting or gambling debt but these terms can be applied to other contexts as well. They are considered derogatory.

welch (n)

A person who defaults on an obligation, especially a small one.

She's a welch. That isn't hers, I lent her that watering can three years ago.

welch (v)

1. To fail to repay a small debt.
2. To fail to fulfill an obligation.

http://en.wiktionary.org/wiki/welch

Related questions:

• Are the terms "welsh" or "welch" (as in reneging on a bet) derogatory toward the Welsh people?
• What would you call someone who imposes on other people's generosity?
• What do you call someone who is always asking for favours?