From the RadioLab podcast linked above (excellent, btw), far more frightening in the long run: introduce genetic changes to the reproductive cells.
... which then automatically become part of any offspring.
... which are then automatically inherited by any subsequent generations.
And, new species.
In my mind the big risk comes with home based DNA printers. There are several close to getting to market (eg http://www.kilobaser.com/), at that point we lose control over what gets printed and then maybe there are concerns... though I do think creating a pathogen is really hard and most likely to end up killing the creator before anyone else.
You have never had control. Perhaps instead you are more worried about our biological weaknesses; there are many! People die all the time from various diseases and even aging. I suggest fixing this before you outlaw DNA manipulation. All of life is a manipulation of DNA in one way or another, and is in fact essential to the maintenance of life... But vulnerabilities should be patched, not swept under the carpet.
While it's certainly true that it's hard to determine the function of a DNA sequence from scratch, it's considerably easier to compare that sequence (or the sequence of the translated polypeptide) to other homologous sequences to see if it matches something dangerous.
I previously worked in a lab that studied Bacillus anthracis, and we had a bit of trouble getting a major gene synthesis company [1] to produce a plasmid with a variant of atxA [2], and atxA isn't even a toxin, it's just a transcriptional regulator. We presumed that they just BLASTed [3] the sequence we gave them and threw up a red flag when it matched anthracis. So this sort of sequence-checking already occurs.
I can imagine a lot worse than that. Imagine genetic engineering gets dirt cheap, and that does seem to be the direction we're headed. Novel pathogens are going to be a lot easier to design than treatments and preventative measures to protect against them. How do you stop the proliferation of bio-weapons? I'm thinking that would be about as easy as stopping the proliferation of malware.