It is up to the individual to decide what to think. Sadly, you, in line with most Daily Mail readers, fail to do that.
And here's why.
The Daily Mail recites opinion and presents it as fact in a way that few other papers do. The Mirror is one exception, presenting political news with a left-wing bias by slipping in phrases like 'Tory toff David Cameron'.
More creditable papers reserve opinion for their comment pages - although the Independent publishes bold opinions on its front pages. Unlike the Mail, they don't disguise this as news and are happy to be called a 'viewspaper'.
Yes, papers still retain a political leaning, but none as fervent as that of the Daily Mail - (its frequent bugbears and alarmist reporting was brilliantly skewered by Frankie Boyle who described the perfect Mail headline as 'Immigrants Carry A New Form Of AIDS Which May Harm House Prices')
As for your comment about reporting standards, I'm sure you can see that an agenda can be pursued without resorting to lying? It's perfecly possible to report accuratley but with bias - by selecting to report only those stories that support your intended gripe. Do you see that?
It's publishing motivated not by courage or righteousness, but by fear and hatred. Suits you, sir.