Trade unions have deviated from their principles.why it is a curse for the modern workers?

I certainly agree with your statement.  I grew up in an
automobile industry city.  At one time, one of the big three American automakers
employed over 30,000 people just in my hometown.  As times became more difficult, plants
closed and people lost their jobs, but most from my hometown agree that the unemployment
is not solely the result of tough times.  While there was certainly a need for trade
unions in the 1940s/50s, the unions (especially the UAW in this case) eventually forced
companies to pay unskilled workers such high wages and offer unbelievably good benefits
that the companies cannot sustain those costs while still makinga profit.  In the 90s
when I was in high school, many of my classmates' fathers worked in the car factories
as assembly line workers and made an annual salary of $70,000--that's an incredible
amount of money for someone with only a high school diploma!  We witnessed last
year that GM is virtually powerless to make the necessary changes that they desperately
need to make in order to turn a profit.  Instead of looking out for their average union
members, the UAW bargained and fought against changes that would truly keep more jobs
open and put the company back on track. 

In addition to the
specific example above, trade unions have also hurt the modern worker
by:

--strongarming them to vote for specific candidates or
parties (ones who agree with union doctrine)

--forcing
workers to lose pay and work time over issues that the workers might not have
individually chosen to strike over.

--causing companies to
move their plants/branches to other states or even other countries where unions do not
have control.